6/30/06
TRANSPORTATION RESEARCH BOARD SPECIAL REPORT 2 8 6 TIRES AND
PASSENGER VEHICLE FUEL ECONOMY
Informing Consumers, Improving Performance
T R B S P E C I A L R E P O R T 2 8 6
Tires and Passenger
Vehicle Fuel
Economy
Informing Consumers, Improving Performance
Committee for the National Tire Efficiency Study
Transportation Research Board
Board on Energy and Environmental Systems
NATIONAL RESEARCH COUNCIL
OF THE NATIONAL ACADEMIES
Transportation Research Board
Washington, D.C.
2006
www.TRB.org
Transportation Research Board Special Report 286
Subscriber Category
IB energy and environment
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Copyright 2006 by the National Academy of Sciences. All rights reserved. Printed in the United States of America.
: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competencies and with regard for appropriate balance.
This report has been reviewed by a group other than the authors according to the procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.
This report was sponsored by the National Highway Traffic Safety Administration of the U.S. Department of Transportation.
Library of Congress Cataloging-in-Publication Data
Tires and passenger vehicle fuel economy : informing consumers, improving performance /
Committee for the National Tire Efficiency Study, Transportation Research Board of the National Academies.
p. cm.—(Special report / Transportation Research Board of the National Academies ;
286)
ISBN 0-309-09421-6
1. Transportation, Automotive—United States. 2. Automobiles—Tires. 3. Automobiles—Fuel consumption. 4. Consumer education—United States. I. National Research Council (U.S.). Transportation Research Board. Committee for the National Tire Efficiency Study. II. Special report (National Research Council (U.S.). Transportation Research Board) ; 286.
HE5623.T57 2006
629.2′482—dc22
2006044478
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. On the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences.
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The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both the Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. William A. Wulf are chair and vice chair, respectively, of the National Research Council.
The Transportation Research Board is a division of the National Research Council, which serves the National Academy of Sciences and the National Academy of Engineering. The Board’s mission is to promote innovation and progress in transportation through research. In an objective and interdisciplinary setting, the Board facilitates the sharing of information on transportation practice and policy by researchers and practitioners; stimulates research and offers research management services that promote technical excellence; provides expert advice on transportation policy and programs; and disseminates research results broadly and encourages their implementation. The Board’s varied activities annually engage more than 5,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. www.TRB.org
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Transportation Research Board 2006 Executive Committee*
Chair: Michael D. Meyer, Professor, School of Civil and Environmental
Engineering, Georgia Institute of Technology, Atlanta
Vice Chair: Linda S. Watson, Executive Director, LYNX-Central Florida
Regional Transportation Authority, Orlando
Executive Director: Robert E. Skinner, Jr., Transportation Research Board
Michael W. Behrens, Executive Director, Texas Department of
Transportation, Austin
Allen D. Biehler, Secretary, Pennsylvania Department of Transportation,
Harrisburg
D. Bowe, Regional President, APL Americas, Oakland, California Larry L. Brown, Sr., Executive Director, Mississippi Department of
Transportation, Jackson
Deborah H. Butler, Vice President, Customer Service, Norfolk Southern
Corporation and Subsidiaries, Atlanta, Georgia
Anne P. Canby, President, Surface Transportation Policy Project,
Washington, D.C.
G. Duncan, President and CEO, FedEx Freight, Memphis, Tennessee Nicholas J. Garber, Henry L. Kinnier Professor, Department of Civil
Engineering, University of Virginia, Charlottesville
Angela Gittens, Vice President, Airport Business Services, HNTB
Corporation, Miami, Florida
Giuliano, Professor and Senior Associate Dean of Research and
Technology, School of Policy, Planning, and Development, and Director, METRANS National Center for Metropolitan Transportation Research, University of Southern California, Los Angeles (Past Chair, 2003)
Susan Hanson, Landry University Professor of Geography, Graduate School
of Geography, Clark University, Worcester, Massachusetts James R. Hertwig, President, CSX Intermodal, Jacksonville, Florida Gloria J. Jeff, General Manager, City of Los Angeles Department of
Transportation, California
Adib K. Kanafani, Cahill Professor of Civil Engineering, University of
California, Berkeley
Harold E. Linnenkohl, Commissioner, Georgia Department of
Transportation, Atlanta
Sue McNeil, Professor, Department of Civil and Environmental Engineering,
University of Delaware, Newark
L. Miller, Secretary, Kansas Department of Transportation, Topeka Michael R. Morris, Director of Transportation, North Central Texas Council
of Governments, Arlington
Carol A. Murray, Commissioner, New Hampshire Department of
Transportation, Concord
John R. Njord, Executive Director, Utah Department of Transportation,
Salt Lake City (Past Chair, 2005)
Sandra Rosenbloom, Professor of Planning, University of Arizona, Tucson
*Membership as of May 2006.
Henry Gerard Schwartz, Jr., Senior Professor, Washington University,
St. Louis, Missouri
Michael S. Townes, President and CEO, Hampton Roads Transit, Virginia
(Past Chair, 2004)
C. Michael Walton, Ernest H. Cockrell Centennial Chair in Engineering,
University of Texas, Austin
Marion C. Blakey, Administrator, Federal Aviation Administration,
U.S. Department of Transportation (ex officio)
Joseph H. Boardman, Administrator, Federal Railroad Administration,
U.S. Department of Transportation (ex officio)
Rebecca M. Brewster, President and COO, American Transportation
Research Institute, Smyrna, Georgia (ex officio)
George Bugliarello, Chancellor, Polytechnic University of New York,
Brooklyn; Foreign Secretary, National Academy of Engineering, Washington, D.C. (ex officio)
Sandra K. Bushue, Deputy Administrator, Federal Transit Administration,
U.S. Department of Transportation (ex officio)
J. Richard Capka, Acting Administrator, Federal Highway Administration,
U.S. Department of Transportation (ex officio)
Thomas H. Collins (Adm., U.S. Coast Guard), Commandant, U.S. Coast
Guard, Washington, D.C. (ex officio)
James J. Eberhardt, Chief Scientist, Office of FreedomCAR and Vehicle
Technologies, U.S. Department of Energy (ex officio)
Glassman, Deputy Administrator, National Highway Traffic
Safety Administration, U.S. Department of Transportation (ex officio) Edward R. Hamberger, President and CEO, Association of American
Railroads, Washington, D.C. (ex officio)
E. Hoemann, Deputy Administrator, Federal Motor Carrier Safety
Administration, U.S. Department of Transportation (ex officio) John C. Horsley, Executive Director, American Association of State Highway
and Transportation Officials, Washington, D.C. (ex officio) John E. Jamian, Acting Administrator, Maritime Administration,
U.S. Department of Transportation (ex officio)
J. Edward Johnson, Director, Applied Science Directorate, National
Aeronautics and Space Administration, John C. Stennis Space Center, Mississippi (ex officio)
G. Kaveeshwar, Administrator, Research and Innovative Technology
Administration, U.S. Department of Transportation (ex officio) Brigham McCown, Deputy Administrator, Pipeline and Hazardous Materials
Safety Administration, U.S. Department of Transportation (ex officio) William W. Millar, President, American Public Transportation Association,
Washington, D.C. (ex officio) (Past Chair, 1992)
Suzanne Rudzinski, Director, Transportation and Regional Programs,
U.S. Environmental Protection Agency (ex officio)
Jeffrey N. Shane, Under Secretary for Policy, U.S. Department of
Transportation (ex officio)
Carl A. Strock (Maj. Gen., U.S. Army), Chief of Engineers and Commanding
General, U.S. Army Corps of Engineers, Washington, D.C. (ex officio)
Board on Energy and Environmental Systems
Douglas M. Chapin, MPR Associates, Inc., Alexandria, Virginia, Chair Robert W. Fri, Resources for the Future (senior fellow emeritus),
Washington, D.C., Vice Chair
J. Bard, University of Texas, Austin David L. Bodde, Clemson University, South Carolina Philip R. Clark, GPU Nuclear Corporation (retired), Boonton,
New Jersey
E. Linn Draper, Jr., American Electric Power, Inc. (emeritus),
Austin, Texas
Goodman, Southern Company, Birmingham, Alabama David G. Hawkins, Natural Resources Defense Council,
Washington, D.C.
A. Krebs, California Energy Commission, Sacramento Gerald L. Kulcinski, University of Wisconsin, Madison David K. Owens, Edison Electric Institute, Washington, D.C. William F. Powers, Ford Motor Company (retired), Ann Arbor,
Michigan
Prophet, Carrier Corporation, Farmington, Connecticut Michael P. Ramage, ExxonMobil Research & Engineering Company
(retired), Moorestown, New Jersey
Edward S. Rubin, Carnegie Mellon University, Pittsburgh,
Pennsylvania
Maxine Savitz, Honeywell, Inc. (retired), Los Angeles, California Philip R. Sharp, Harvard University, Cambridge, Massachusetts Scott W. Tinker, University of Texas, Austin
Committee for the National Tire Efficiency Study
Dale F. Stein, Michigan Technological University (retired),
Tucson, Arizona, Chair
James E. Bernard, Iowa State University, Ames
John Eagleburger, Goodyear Tire Company (retired), North Canton,
Ohio
J. Farris, University of Massachusetts, Amherst David Friedman, Union of Concerned Scientists, Washington, D.C. Patricia S. Hu, Oak Ridge National Laboratory, Knoxville, Tennessee Wolfgang G. Knauss, California Institute of Technology, Pasadena Christopher L. Magee, Massachusetts Institute of Technology,
Cambridge
G. Pottinger, M’gineering, LLC, Akron, Ohio Karl J. Springer, Southwest Research Institute (retired), San Antonio,
Texas
Margaret A. Walls, Resources for the Future, Washington, D.C. Joseph D. Walter, University of Akron, Ohio
Project Staff
Thomas R. Menzies, Jr., Study Director, Transportation
Research Board
James Zucchetto, Board on Energy and Environmental Systems,
Division on Engineering and Physical Sciences
Preface
In February 2005, in response to a congressional request1 and with fund
ing from the National Highway Traffic Safety Administration (NHTSA) of the U.S. Department of Transportation, the National Research Council (NRC) formed the Committee for the National Tire Efficiency Study. The committee consisted of 12 members with expertise in tire engineering and manufacturing, mechanical and materials engineering, and statistics and economics.
The committee was given the following charge:
This study will develop and perform a national tire efficiency study and literature review to:
• Consider the relationship that low rolling resistance replacement tires
designed for use on passenger cars and light trucks have on fuel consumption and tire wear life;
• Address the potential for securing technically feasible and cost-effective
replacement tires that do not adversely affect safety, including the impacts on performance and durability, or adversely impact tire tread life and scrap tire disposal;
• Fully consider the average American “drive cycle” in its analysis; • Address the cost to the consumer including the additional cost of
replacement tires and any potential fuel savings.
In approaching its charge, the committee made a number of decisions affecting the study scope and logic and content of the report.
1 Conference Report 108-401, to Accompany H.R. 2673, Making Appropriations for Agriculture,
Rural Development, Food and Drug Administration, and Related Agencies for the Fiscal Year Ending September 30, 2004, and for Other Purposes. November 25, 2003, p. 971.
i x
x Tires and Passenger Vehicle Fuel Economy
These decisions are explained in Chapter 1. For the most part, the committee sought to answer each of the questions asked by Congress by examining the technical literature and available data on passenger tire performance characteristics.
The committee met four times between April and October 2005 and communicated extensively by e-mail and teleconference. Meetings included open sessions for gathering information from outside experts from industry, government, and academia, as well as closed deliberative sessions for discussions among committee members. In addition, selected committee members, staff, and consultants met with representatives of automobile manufacturers and experts in tire materials and technologies between committee meetings.
Before the committee’s final meeting, several tire manufacturers, acting through the Rubber Manufacturers Association, made available measurements of the rolling resistance of a sample of more than 150 new replacement passenger tires as well as some original equipment (OE) tires. Although the sample was not scientifically derived, the data proved helpful to the committee as it sought to answer the various questions in the study charge. The timing of the data’s availability late in the study process limited the statistical analyses that could be undertaken by the committee. Nevertheless, the committee appreciates the efforts of Michelin North America, Bridgestone Americas, and the Goodyear Tire and Rubber Company in providing these data as requested.
ACKNOWLEDGMENTS
During the course of its deliberations, the committee benefited from presentations and information provided by the following individuals, whom the committee acknowledges and thanks: Ronald Medford, Joseph Kanianthra, and W. Riley Garrott, NHTSA; Lois Platte, U.S. Environmental Protection Agency; Luke Tonachel, National Resources Defense Council; Donald Shea, Tracey Norberg, and Michael Blumenthal, Rubber Manufacturers Association; Arnold Ward, California Energy Commission; Christopher Calwell, Ecos Consulting, Inc.; Ed Cohn, California Tire Dealers Association—South; Mitchell Delmage, California Integrated Waste Management Board; Andrew Burke, Daniel Sperling, Paul Erickson, Andrew Frank, and Christopher Yang, Institute of
Preface xi
Transportation Studies, University of California, Davis; Terry Laveille, California Tire Report; Donald Amos, Continental Tire North America; Anthony Brinkman, Cooper Tire and Rubber Company; Georg Böhm (retired) and Dennis Candido, Bridgestone Americas; Simeon Ford, Goodyear Tire and Rubber Company; Michael Wischhusen, Michelin North America; Paul Daniels, Pirelli Tire North America; and Alan McNeish, Degussa (retired).
The committee is grateful to Jonathan Mueller and James MacIsaac of NHTSA for serving as the agency’s technical liaisons to the study. Special thanks go to Daniel Sperling, Andrew Burk, Alexis Palecek, and other staff, faculty, and students of the Institute of Transportation Studies at the University of California, Davis, which hosted the committee’s second meeting. Thanks also go to Guy Edington, Director of the Kumho Tire Technical Center, which hosted a subcommittee meeting in Akron, Ohio, and to Douglas Domeck, James Popio, James McIntyre, and other staff of Smithers Scientific Service, Inc., which provided a tour of the company’s Transportation Test Center in Ravenna, Ohio. In addition, the committee is grateful to Ed Noga of Rubber and Plastics News, which provided Internet access to its archives, and to John Smith of Standard Testing Laboratories for providing tire section cutaways and other presentation aids.
Thomas R. Menzies, Jr., managed the study and drafted the final report under the guidance of the committee and the supervision of Stephen R. Godwin, Director of Studies and Information Services. Committee member Marion G. Pottinger drafted the Appendix, and committee member Margaret A. Walls conducted the multiple regression analyses in Chapters 3 and 4. The committee was aided by consultant K. G. Duleep of Energy and Environmental Analysis, Inc. He interviewed automobile manufacturers to learn about their interest in the rolling resistance characteristics of OE tires. He also provided the committee with analyses of the influence of passenger tires on motor vehicle fuel economy.
The report was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise in accordance with procedures approved by the NRC’s Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the institution in making its published report as sound as
xii Tires and Passenger Vehicle Fuel Economy
possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process.
The committee thanks the following individuals for their review of this report: Karin M. Bauer, Midwest Research Institute, Kansas City, Missouri; Nissim Calderon, the Goodyear Tire and Rubber Company (retired), Boca Raton, Florida; W. Dale Compton, Purdue University, West Lafayette, Indiana; Alan N. Gent, University of Akron, Ohio; Thomas D. Gillespie, University of Michigan Transportation Research Institute, Ann Arbor; Marc H. Ross, University of Michigan (Emeritus), Ann Arbor; Nicholas M. Trivisonno, B. F. Goodrich (retired), Broadview Heights, Ohio; and Sarah E. West, Macalester College, St. Paul, Minnesota. Although these reviewers provided many constructive comments and suggestions, they were not asked to endorse the committee’s findings and conclusions, nor did they see the final report before its release. The review of this report was overseen by Maxine L. Savitz, Honeywell International, Inc. (retired), and C. Michael Walton, University of Texas at Austin. Appointed by NRC, they were responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.
Suzanne Schneider, Associate Executive Director of the Transportation Research Board, managed the report review process. The report was edited and prepared for publication by Norman Solomon, Senior Editor, and the final manuscript was formatted and prepared for initial release and web posting by Jennifer J. Weeks, under the supervision of Javy Awan, Director of Publications. Special thanks go to Frances Holland and Amelia Mathis for assistance with meeting arrangements and correspondence with the committee.
Abbreviations and Glossary
Aspect ratio. A tire’s section height divided by its section width, multiplied by 100. Aspect ratio is listed in the size designation on the passenger tire sidewall. Typical tire aspect ratios range from 35 for tires used on sports cars to 75 for tires used on utility-type vehicles.
Bead. A ring of steel wire that anchors the tire carcass plies to the rim.
Belt. An assembly of plies extending from shoulder to shoulder of a tire and providing a reinforcing foundation for the tread. In radial-ply tires, the belts are typically reinforced with fine steel wire having high tensile strength.
Bias-ply tire. A pneumatic tire in which the ply cords that extend to the beads are laid at alternate angles substantially less than 90 degrees to the centerline of the tread. The bias-ply tire was the predominant passenger tire in the United States before 1980 but is no longer in common use; it has been supplanted by the radial-ply tire.
Carbon black. A very fine, nano-size particulate carbon used as a reinforcing filler in rubber compounds to provide abrasion resistance and other favorable properties.
Carcass or casing. The tire structure, except tread and sidewall rubber, that bears the load when the tire is inflated.
Coastdown. A process in which a vehicle or test machine is allowed to slow down freely from a high to a low speed without application of external power or braking.
x i i i
xiv Tires and Passenger Vehicle Fuel Economy
Coefficient of friction. The ratio of friction force to normal force to cause sliding expressed as a unitless value (i.e., friction force generated between tire tread rubber and the road surface divided by vertical load).
Corporate average fuel economy (CAFE). A federal program that sets a minimum performance requirement for passenger vehicle fuel economy. Each automobile manufacturer must achieve an average level of fuel economy for all specified vehicles manufactured in a given model year. The National Highway Traffic Safety Administration administers the CAFE program. The U.S. Environmental Protection Agency develops the vehicle fuel economy test procedures.
EPA. U.S. Environmental Protection Agency. EPA is responsible for developing the federal test procedures for measuring and rating the fuel economy of new passenger cars and light trucks. The federal test procedures are used for new vehicle fuel economy labeling and the corporate average fuel economy program.
FMVSS. Federal Motor Vehicle Safety Standards. The FMVSS include regulations governing passenger tire safety.
High-performance tire. A passenger tire designed for the highest speed and handling, generally having the speed symbol W, Y, or Z in the United States.
Hysteresis. A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery. The rubber compound in a tire exhibits hysteresis. As the tire rotates under the weight of the vehicle, it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat. Hysteresis is the main cause of energy loss associated with rolling resistance and is attributed to the viscoelastic characteristics of the rubber.
Light truck (LT) tire. A tire constructed for heavy loads and rough terrain that is usually used on medium-duty trucks in commercial service. These tires contain the prefix LT before the metric size designation
Abbreviations and Glossary xv
molded on the tire sidewall and are inflated to higher pressures than are normal passenger tires. LT tires are not regulated as passenger tires and are therefore not examined in this study.
NHTSA. National Highway Traffic Safety Administration. Among its responsibilities, NHTSA administers the Federal Motor Vehicle Safety Standards, the Uniform Tire Quality Grading system, and the corporate average fuel economy program.
Original equipment manufacturer (OEM). An automobile manufacturer.
Original equipment (OE) passenger tire. A tire that is provided as original equipment on new passenger vehicles. Such tires are often designed for particular vehicles to the specifications of the automobile manufacturer.
Passenger tire. A tire constructed and approved for use on passenger vehicles and that usually contains the prefix P before the metric size designation on the tire sidewall. Federal Motor Vehicle Safety Standards and Uniform Tire Quality Grading standards are established specifically for passenger tires.
Passenger vehicle. For the purposes of this report, a car or light truck used primarily for passenger transportation. Most of these vehicles use passenger tires. Most vans, pickup trucks, and sport utility vehicles that are categorized as light trucks by the federal government are considered passenger vehicles. Light trucks that exceed 6,000 pounds in gross vehicle weight are usually used for nonpassenger commercial service. They are usually equipped with light truck (LT) tires.
Performance tire. A passenger tire intended to provide superior handling and higher speed capabilities and generally having a speed symbol of H or V in the United States.
Ply. A sheet of rubber-coated parallel tire cords. Tire body plies are layered.
xvi Tires and Passenger Vehicle Fuel Economy
Radial-ply construction. A pneumatic tire construction under which the ply cords that extend to the beads are laid at approximately 90 degrees to the centerline of the tread. Two or more plies of reinforced belts are applied, encircling the tire under the tread. Radial-ply tires were introduced in Europe during the 1950s and came into common use in the United States during the 1970s.
Reinforcing filler. Material added to rubber compounds to provide favorable properties, including resistance to abrasion. The two most common reinforcing fillers are carbon black and silica.
Replacement passenger tire. A tire purchased in the aftermarket to replace an original equipment tire.
Rim diameter. The diameter of a wheel measured at the intersection of the bead seat and the flange. The rim diameter is listed in the size designation on the passenger tire sidewall. Common rim diameters for passenger tires range from 13 to 20 inches.
RMA. Rubber Manufacturers Association. RMA is the national trade association for the rubber products industry in the United States. Most domestic and foreign tire makers who produce tires in the United States are members of the association.
Rolling resistance. The force at the axle in the direction of travel required to make a loaded tire roll.
Rolling resistance coefficient (RRC). The value of the rolling resistance force divided by the wheel load. The Society of Automotive Engineers (SAE) has developed test practices to measure the RRC of tires. These tests (SAE J1269 and SAE J2452) are usually performed on new tires. When measured by using these standard test practices, most new passenger tires have reported RRCs ranging from 0.007 to 0.014.
Run-flat tire. A type of pneumatic tire constructed of special materials, supports, and configurations that allow it to travel for a limited distance
Abbreviations and Glossary xvii
and speed after experiencing a loss of most or all inflation pressure. While these tires usually have greater weight and resultant rolling resistance, they permit the elimination of storage space and weight associated with a spare tire and jack.
SAE. Society of Automotive Engineers. SAE technical committees have developed standardized test practices for measuring the rolling resistance of tires.
SAE J1269. A recommended practice of SAE that defines a standardized method for testing tire rolling resistance under steady-state conditions at 80 km/h (50 mph).
SAE J2452. A recommended practice of SAE that defines a standardized method for testing tire rolling resistance in simulation of a coastdown from 120 to 15 km/h.
Section height. The linear distance between an inflated unloaded tire’s overall (outside) tread diameter and the intersection of the bead seat and the flange.
Section width. The linear distance between the outside sidewalls of an inflated unloaded tire (not including decorations such as lettering) when mounted on the measuring rim. Treads are always narrower than the section width.
Sidewall. The portion of the tire between the bead and the tread. The tire’s name, safety codes, and size designation are molded on the sidewall.
Silane. An organo-silicate compound that is sometimes mixed with silica to promote dispersion and bonding.
Silica. A very fine, nano-size particle, silicon dioxide, used as a reinforcing filler in rubber compounding.
Speed rating. A letter assigned to a tire denoting the maximum speed for which the use of the tire is rated (e.g., S = 112 mph, H = 130 mph).
xviii Tires and Passenger Vehicle Fuel Economy
The speed rating is contained in the tire size designation molded on the sidewall.
Tire pressure monitoring system (TPMS). A warning system in motor vehicles that indicates to the operator when a tire is significantly underinflated. Some systems use sensors in the tire to transmit pressure information to a receiver. Some do not have pressure sensors but rely on wheel speed sensors to detect and compare differences in wheel rotational speeds, which can be correlated to differences in tire pressure.
Traction. The ability of a loaded tire to generate vehicle control forces through frictional interaction with a road surface.
Tread. The peripheral portion of the tire designed to contact the road surface. The tread band consists of a pattern of protruding ribs and grooved channels on top of a base. Tread depth is measured on the basis of groove depth. Traction is provided by the tread.
Tread compound. The general term that refers to the chemical formula of the tread material. The compound consists of polymers, reinforcing fillers, and other additives that aid in processing and slow degradations from heat, oxygen, moisture, and ozone.
Tread wear life. Total miles traveled by a tire until its tread wears out, which is usually defined as a remaining groove depth of 2/32 inch for a passenger car tire that exhibits even wear.
Uniform Tire Quality Grade (UTQG). A passenger tire rating system that grades a tire’s performance in tread wear durability, traction, and temperature resistance. UTQG ratings are required by the federal government for most types of passenger tires and are molded on the tire’s sidewall. The tread wear grade is a numeric rating, with a higher number suggesting longer tread wear capability. Most tires receive grades between 100 and 800. The traction grade is assigned on the basis of results of skid tests on wet pavements. Tires are graded AA, A, B, or C, with AA indicating superior wet traction. The temperature grade is assigned to
Abbreviations and Glossary xix
tires tested at various speeds to determine the ability of a tire to dissipate heat. Tires are graded A, B, or C, with A indicating an ability to dissipate heat at higher speeds.
USDOT. U.S. Department of Transportation. The National Highway Traffic Safety Administration is an agency of USDOT.
Vehicle fuel economy. The average number of miles a vehicle travels per gallon of motor fuel (typically gasoline or diesel fuel).
Viscoelastic. A viscoelastic material is characterized by possessing both viscous and elastic behavior. A purely elastic material is one in which all energy stored in the material during loading is returned when the load is removed. In contrast, a purely viscous material stores no strain energy, and all of the energy required to deform the material is simultaneously converted into heat. Some of the energy stored in a viscoelastic system is recovered on removal of the load, and the remainder is dissipated as heat. Rubber is a viscoelastic material.
Wear resistance. Resistance of the tread to abrasion from use on a normal road surface.
Wet traction. The ability of a loaded tire to generate vehicle control forces through frictional interaction with a wet road surface.
Contents
Executive Summary 1
1 Introduction 9
Study Charge and Scope 10
Policy Context 11
Study Approach and Information Base 13
Report Organization 16
2 Background on Passenger Tires 17
Tire Terminology and Trends 17
History of Tire Development 20
Tire Industry Structure 26
Tire Safety and Consumer Information Standards 29
Summary 33
3 The Tire’s Influence on Passenger Vehicle
Fuel Consumption 36
Recent History of Interest in Vehicle Fuel Economy 37
Examining the Influence of Tires on Vehicle Fuel Economy 39
Factors Causing and Influencing Rolling Resistance 42
Measuring and Expressing Rolling Resistance 47
Rolling Resistance and Fuel Economy 49
Rolling Resistance Data for Passenger Tires 51
Summary 73
4 Rolling Resistance, Traction, and Wear
Performance of Passenger Tires 77
Effects on Traction and Safety Performance 79
Effects on Tread Life and Scrap Tires 88
Summary 102
5 National Consumer Savings and Costs 105
Consumer Fuel Savings 107
Consumer Tire Expenditures 108
Overall Effect on Consumer Expenditures 119
6 Findings, Conclusions, and Recommendations 123
Key Findings and Estimates 124
Conclusions in Response to Study Charge 131
Recommendations to Inform Consumers 134
APPENDIX: Explanation and Comparison of
Society of Automotive Engineers Test Procedures
for Rolling Resistance 137
Marion G. Pottinger
Study Committee Biographical Information 146
Executive Summary
Each year Americans spend about $20 billion replacing the tires on their passenger cars and light trucks. Although passenger tires last far longer today than they did 30 years ago, most are replaced every 3 to 5 years because of wear. A total of about 200 million replacement passenger tires are purchased in the United States annually. Each time they replace their tires, motorists spend several hundred dollars and must choose among tires varying in price, style, and many aspects of performance. The tires they do buy will affect not only the handling, traction, ride comfort, and appearance of their vehicles but also fuel economy.
Tires affect vehicle fuel economy mainly through rolling resistance. As a tire rolls under the vehicle’s weight, its shape changes repeatedly as it experiences recurring cycles of deformation and recovery. In the process, mechanical energy otherwise available to turn the wheels is converted into heat and dissipated from the tire. More fuel must be expended to replace this lost energy. Combinations of differences in tire dimensions, design, materials, and construction features will cause tires to differ in rolling resistance as well as in many other attributes such as traction, handling, noise, wear resistance, and appearance. Once they are placed in service, tires must be properly maintained to perform as intended with respect to all attributes. The maintenance of proper inflation pressure is especially important.
The collective outcomes of the choices consumers make when they buy tires are matters of public interest. The 220 million passenger cars and light trucks in the United States consume about 130 billion gallons of motor fuel annually. Finding ways to reduce this energy consumption is a national goal for reasons ranging from ensuring economic and national security to improving local air quality and reducing greenhouse
2 Tires and Passenger Vehicle Fuel Economy
gas emissions. Maximizing the wear life of tires is also important from the public standpoint of controlling the population of scrap tires that can burden landfills and recycling programs. While the handling, traction, and other operating characteristics of tires are of particular interest to tire buyers, they are also matters of broader public interest inasmuch as they may influence the safety performance of vehicles on the nation’s highways.
This study was conducted at the request of Congress with funding from the National Highway Traffic Safety Administration (NHTSA). It examines the rolling resistance characteristics of passenger tires sold for replacement and how differences in rolling resistance relate to other tire attributes. Specifically, Congress asked the National Research Council (NRC) to assess the feasibility of reducing rolling resistance in replacement tires and the effects of doing so on vehicle fuel consumption, tire wear life and scrap tire generation, and tire operating performance as it relates to motor vehicle safety. Congress asked that the assessment include estimates of the effects of reductions in rolling resistance on consumer spending on fuel and tire replacement.
To conduct the study, the Transportation Research Board, under the auspices of NRC, assembled a committee of experts in tire engineering and manufacturing, mechanical and materials engineering, and statistics and economics. The study committee reviewed the technical literature and analyzed data on passenger tire rolling resistance and other characteristics. Many aspects of tire design, construction, and manufacturing are proprietary, which limits the availability of quantitative information, particularly on the effects of specific changes in tire design and construction to reduce rolling resistance. Nevertheless, enough quantitative and technical information exists in the public domain to assess and reach some general conclusions about the feasibility of reducing rolling resistance in replacement tires and the implications for other tire attributes. Effects on consumer spending on fuel and tire replacement can also be approximated.
The study findings and conclusions are summarized below. Taken together, they persuade the committee that the influence of passenger tires on vehicle fuel consumption warrants greater attention by government, industry, and consumers. A recommendation for congressional action is offered in light of this conclusion.
Executive Summary 3
FEASIBILITY OF LOWERING ROLLING RESISTANCE IN REPLACEMENT TIRES
Reducing the average rolling resistance of replacement tires by a magnitude of 10 percent is technically and economically feasible. A tire’s overall contribution to vehicle fuel consumption is determined by its rolling resistance averaged over its lifetime of use. A reduction in the average rolling resistance of replacement tires in the fleet can occur through various means. Consumers could purchase more tires that are now available with lower rolling resistance, tire designs could be modified, and new tire technologies that offer reduced rolling resistance could be introduced. More vigilant maintenance of tire inflation pressure will further this outcome. In the committee’s view, there is much evidence to suggest that reducing the average rolling resistance of replacement tires by a magnitude of 10 percent is feasible and attainable within a decade through combinations of these means.
Rolling resistance varies widely among replacement tires already on the market, even among tires that are comparable in price, size, traction, speed rating, and wear resistance. Consumers, if sufficiently informed and interested, could bring about a reduction in average rolling resistance by adjusting their tire purchases and by taking proper care of their tires once in service, especially by maintaining recommended inflation pressure. The committee does not underestimate the challenge of changing consumer preferences and behavior. This could be a difficult undertaking, and it must begin with information concerning the tire’s influence on fuel economy being made widely and readily available to tire buyers and sellers. A significant and sustained reduction in rolling resistance is difficult to imagine under any circumstances without informed and interested consumers.
The committee observes that consumers now have little, if any, practical way of assessing how tire choices can affect vehicle economy.
INFLUENCE ON VEHICLE FUEL ECONOMY
Tires and their rolling resistance characteristics can have a meaningful effect on vehicle fuel economy and consumption. A 10 percent reduction in average rolling resistance, if achieved for the population of
4 Tires and Passenger Vehicle Fuel Economy
passenger vehicles using replacement tires, promises a 1 to 2 percent increase in the fuel economy of these vehicles. About 80 percent of passenger cars and light trucks are equipped with replacement tires. Assuming that the number of miles traveled does not change, a 1 to 2 percent increase in the fuel economy of these vehicles would save about 1 billion to 2 billion gallons of fuel per year of the 130 billion gallons consumed by the entire passenger vehicle fleet. This fuel savings is equivalent to the fuel saved by taking 2 million to 4 million cars and light trucks off the road. In this context, a 1 to 2 percent reduction in the fuel consumed by passenger vehicles using replacement tires would be a meaningful accomplishment.
EFFECTS ON TIRE WEAR LIFE AND SCRAP TIRES
The effects of reductions in rolling resistance on tire wear life and scrap tires are difficult to estimate because of the various ways by which rolling resistance can be reduced. The tread is the main factor in tire wear life and the main component of the tire contributing to rolling resistance. Reductions in tread thickness, volume, and mass are among the means available to reduce rolling resistance, but they may be undesirable if they lead to shorter tire lives and larger numbers of scrap tires. Various tread-based technologies are being developed and used with the goal of reducing rolling resistance without significant effects on wear resistance. The practical effects of these technologies on tread wear and other tire performance characteristics have not been established quantitatively. However, continuing advances in tire technology hold much promise that rolling resistance can be reduced further without adverse effects on tire wear life and scrap tire populations.
EFFECTS ON TRACTION AND SAFETY PERFORMANCE
Although traction may be affected by modifying a tire’s tread to reduce rolling resistance, the safety consequences are probably undetectable. Changes are routinely made in tire designs, materials, and construction methods for reasons ranging from noise mitigation and ride comfort to steering response and styling. All can have implications for other tire
Executive Summary 5
properties and operating performance, including traction capability. Discerning the safety implications of small changes in tire traction characteristics associated with tread modifications to reduce rolling resistance may not be practical or even possible. The committee could not find safety studies or vehicle crash data that provide insight into the safety impacts associated with large changes in traction capability, much less the smaller changes that may occur from modifying the tread to reduce rolling resistance.
EFFECTS ON CONSUMER FUEL AND TIRE EXPENDITURES
Reducing the average rolling resistance of replacement tires promises fuel savings to consumers that exceed associated tire purchase costs, as long as tire wear life is not shortened. A 10 percent reduction in rolling resistance can reduce consumer fuel expenditures by 1 to 2 percent for typical vehicles. This savings is equivalent to 6 to 12 gallons per year, or $12 to $24 if fuel is priced at $2 per gallon. Tire technologies available today to reduce rolling resistance would cause consumers to spend slightly more when they buy replacement tires, on the order of 1 to 2 percent or an average of $1 to $2 more in tire expenditures per year. These technologies, however, may need to be accompanied by other changes in tire materials and designs to maintain the levels of wear resistance that consumers demand. While the effect of such accompanying changes on tire production costs and prices is unclear, the overall magnitude of the fuel savings suggests that consumers would likely incur a net savings in their combined fuel and tire expenditures.
RECOMMENDATIONS TO INFORM CONSUMERS
As a general principle, consumers benefit from the ready availability of easyto-understand information on all major attributes of their purchases. Tires are no exception, and their influence on vehicle fuel economy is an attribute that is likely to be of interest to many tire buyers. Because tires are driven tens of thousands of miles, their influence on vehicle fuel consumption can extend over several years. Ideally, consumers would have access to information that reflects a tire’s effect on fuel economy averaged over its
6 Tires and Passenger Vehicle Fuel Economy
anticipated lifetime of use, as opposed to a measurement taken during a single point in the tire’s lifetime, usually when it is new. No standard measure of lifetime tire energy consumption is currently available, and the development of one deserves consideration. Until such a practical measure is developed, rolling resistance measurements of new tires can be informative to consumers, especially if they are accompanied by reliable information on other tire characteristics such as wear resistance and traction. Advice on specific procedures for measuring and rating the influence of individual passenger tires on fuel economy and methods of conveying this information to consumers is outside the scope of this study. Nevertheless, the committee is persuaded that there is a public interest in consumers having access to such information. The public interest is comparable with that of consumers having information on tire traction and tread wear characteristics, which is now provided by industry and required by federal regulation.
It is apparent that industry cooperation is essential in gathering and conveying tire performance information that consumers can use in making tire purchases. It is in the spirit of prompting and ensuring more widespread industry cooperation in the supply of useful and trusted purchase information that the committee makes the following recommendations.
Congress should authorize and make sufficient resources available to NHTSA to allow it to gather and report information on the influence of individual passenger tires on vehicle fuel consumption. Information that best indicates a tire’s contribution to vehicle fuel consumption and that can be effectively gathered, reported, and communicated to consumers buying tires should be sought. The effort should cover a large portion of the passenger tires sold in the United States and be comprehensive with regard to popular tire sizes, models, and types, both imported and domestic.
NHTSA should consult with the U.S. Environmental Protection Agency on means of conveying the information and ensure that the information is made widely available in a timely manner and is easily understood by both buyers and sellers. In the gathering and communication of this information, the agency should seek the active participation of the entire tire industry.
Executive Summary 7
The effectiveness of this consumer information and the methods used for communicating it should be reviewed regularly. The information and communication methods should be revised as necessary to improve effectiveness. Congress should require periodic assessments of the initiative’s utility to consumers, the level of cooperation by industry, and the resultant contribution to national goals pertaining to energy consumption.
Finally, even as motorists are advised of the energy performance of tires, they must appreciate that all tires require proper inflation and maintenance to achieve their intended levels of energy, safety, wear, and operating performance. As new technologies such as tire pressure monitoring systems, more energy-efficient tire designs, and run-flat constructions are introduced on a wider basis, they must have the effect of prompting more vigilant tire maintenance rather than fostering more complacency in this regard. Motorists must be alerted to the fact that even small losses in inflation pressure can greatly reduce tire life, fuel economy, safety, and operating performance. A strong message urging vigilant maintenance of inflation must therefore be a central part of communicating information on the energy performance of tires to motorists.
1
Introduction
During 2005, gasoline and diesel prices, adjusted for inflation, rose to levels not experienced in the United States in a quarter century. For a growing number of Americans, the price of motor fuel has become a real financial concern. Whether fuel prices will stabilize or fluctuate remains to be seen, but one apparent outcome of recent price instability is renewed interest among consumers and policy makers in vehicle fuel economy. Motor vehicles account for about half of the nation’s petroleum usage, and about three-quarters of this fuel goes to the 220 million cars and light-duty trucks in the nation’s passenger vehicle fleet (Davis and Diegel 2004, 1-17, 1-18, 3-7, 4-2, 4-3).1 In traveling some 2,600 billion miles, these vehicles burn about 130 billion gallons of gasoline and diesel fuel each year, or about 600 gallons per vehicle on average (Davis and Diegel 2004, 4-2, 4-3). In terms of fuel economy, passenger vehicles in the fleet average about 20 miles per gallon (mpg), which includes the 22.1 mpg averaged by cars and the 17.6 mpg averaged by light trucks (Davis and Diegel 2004, 4-2, 4-3).
Many variables affect vehicle fuel economy, among them the vehicle’s weight, aerodynamics, engine, driveline, and accessory load. The vehicle’s tires also influence fuel economy by causing rolling resistance, which consumes energy and thus reduces fuel economy. Anyone who has pedaled a bicycle with tires low on air can attest to the added work required to overcome the increase in rolling resistance. Even if it is prop
1 Statistics on passenger vehicle populations, travel, and motor fuel use referenced in this report are
drawn from the U.S. Department of Energy’s Transportation Energy Data Book, which is cited as Davis and Diegel 2004. The statistics in the Data Book are derived from several sources, including Highway Statistics, published annually by the Federal Highway Administration of the U.S. Department of Transportation. The data are for 2002 and 2003, which were the most recent years available for these statistics when this report was prepared.
1 0 Tires and Passenger Vehicle Fuel Economy
erly inflated, a bicycle tire exhibits rolling resistance that varies with the tire’s size, construction, and materials. This variability, even when slight, can be noticeable to the frequent bicyclist. However, large variations in the rolling resistance of tires used on motor vehicles may go completely unnoticed by the driver, since the vehicle’s engine does all the work. Despite paying the price of more frequent refueling, the driver may never make a connection between the tires and the rate of fuel consumption.
This study examines the contribution of tires to vehicle fuel economy, the variability in energy performance among tires, and technical and economic issues associated with means of improving tire energy performance. The focus is on replacement tires designed for passenger cars as well as vehicles defined as light trucks and used mainly for personal transportation.
Congress requested the study, presumably to help inform both consumers and policy makers. Most motorists will replace their tires every 3 to 5 years, but few are likely to know the effects of their tire purchases on the rate of fuel consumption of their vehicles, because little consumer information is available on this tire characteristic. While the extent of consumer interest in tire energy performance is unclear, it is reasonable to assume that motorists care more about this characteristic when fuel prices are high or rising. With respect to the public interest overall, the approximately 200 million replacement tires that are purchased each year by U.S. consumers have many collective effects on society. Most of the 160 million to 175 million passenger vehicles in the United States that are more than 3 or 4 years old are equipped with replacement tires (Davis and Diegel 2004, 3-9, 3-10). These vehicles make up about 75 percent of the passenger vehicle fleet. Replacement tires thus affect not only motor fuel consumption in the aggregate but also vehicle safety performance and the nation’s solid waste and recycling streams. Consequently, passenger tires have long been the subject of federal, state, and local regulations and environmental policies.
STUDY CHARGE AND SCOPE
Congress requested this study of national tire efficiency. The language of the request, which constitutes the study’s statement of task, can be found in the Preface. In short, Congress called for an evaluation of how lowering
Introduction 1 1
the rolling resistance of replacement tires used on passenger cars and light trucks could affect
• Motor fuel consumption nationally; • Tire wear life and the generation of scrap tires;
• Tire performance characteristics, including those affecting vehicle
safety; and
• Total consumer spending on tires and fuel.
The study request further urges that consideration be given to the “average American drive cycle.” This cycle was not defined, but it suggests that the effects listed above should be considered with ample regard for how tires are used and maintained in practice during their lifetime of service. The request focuses on replacement tires as opposed to original equipment (OE) tires. Replacement tires are purchased directly by consumers, and they are subject to market and regulatory influences different from those of OE tires supplied to automobile manufacturers. The study’s focus on replacement tires, however, does not mean that OE tires are excluded from consideration. Indeed, much can be learned from OE tires. Federal fuel economy regulations that apply to new passenger vehicles have prompted automobile manufacturers to demand tires that will exhibit lower rolling resistance when new equipment on vehicles is subjected to fuel economy testing.2 Moreover, because OE tires are designed
specifically for the vehicles to which they are supplied, motorists may have an interest in replacing them with aftermarket tires that will offer many of the same characteristics and capabilities, including energy performance.
POLICY CONTEXT
A decade ago, the National Highway Traffic Safety Administration (NHTSA) proposed a fuel economy rating for passenger tires—one that would provide tire buyers with a performance grade molded on the tire sidewall.3 Although the rating system was not adopted, the ensuing debate
2 Federal fuel economy standards apply only to new vehicles and do not govern the energy perfor
mance of aftermarket components or maintenance of fuel economy over the lifetime of a vehicle’s operation.
3 59 CFR 19686, 60 CFR 27472, and 61 CFR 47437.
1 2 Tires and Passenger Vehicle Fuel Economy
revealed gaps in the information available concerning tire rolling resistance levels and the effects of lowering rolling resistance on tire wear resistance, other aspects of tire operating performance, and vehicle fuel use. Federal legislative proposals have emerged periodically ever since, including an amendment to the 2005 Energy Policy Act—later withdrawn—calling on NHTSA to establish a national tire efficiency program to set policies and procedures for tire fuel economy testing and labeling and for promoting the sale of replacement tires that consume less energy.
As interest in tire energy performance has fluctuated at the federal level, some state governments and private organizations have taken steps to promote improvements. In 2003, California enacted a law (AB 844) requiring tire manufacturers to report the rolling resistance properties and fuel economy effects of replacement tires sold in the state. Charged with implementing the law, the California Energy Commission, with financial support from the California Integrated Waste Management Board, has been gathering rolling resistance information and other data on passenger tires. The purpose is to assess the feasibility and desirability of establishing a consumer information program or defining an energy performance standard for replacement tires sold in California.
Surprisingly, tire energy performance has received even less attention in Europe and Japan than in the United States. A strong interest in highperformance tires by European and Japanese motorists is one reason for this situation. Nevertheless, since 1977, Germany has administered the “Blue Angel” environmental labeling program, whereby companies voluntarily submit their products for testing and recognition as “environmentally sound.” Passenger tires are one of nearly 100 product categories in the German program, and they are tested for several properties, including noise emissions, wet traction, hydroplaning, and rolling resistance.
Seeking ways to improve the energy performance of individual motor vehicle components, the International Energy Agency (IEA) convened a workshop in November 2005 to examine how rolling resistance is measured in tires and how these measurements can translate into reductions in vehicle fuel consumption. Workshop participants—drawn mostly from Europe and the United States—discussed the grounds for and feasibility of internationally uniform procedures for rating the energy perfor
Introduction 1 3
mance of tires. The IEA activity may be indicative of a growing interest in tire energy performance abroad as well as in the United States.4
STUDY APPROACH AND INFORMATION BASE
Much of the technical literature on tire rolling resistance dates from the mid-1970s to mid-1980s and coincides with rising energy prices and the heightened consumer and government interest in vehicle fuel economy at that time. The studies from that era describe and document the effects of changes in tire designs, dimensions, materials, and operating conditions on rolling resistance. These studies consisted mainly of laboratory experiments and simulations. Much of what is known today about the effects of individual tire components (e.g., tread band, sidewall, and bead) and operating conditions (e.g., tire pressure, vehicle speed, and load) on tire energy performance originated from this earlier period.
Data characterizing the rolling resistance of today’s passenger tires— those on the market and in use on the nation’s highways—are more difficult to obtain. Such data are essential, however, in confirming relationships observed in past experiments and in characterizing rolling resistance levels in the current tire population and their association with other tire performance characteristics. Tires are designed and constructed in several ways that can affect their rolling resistance as well as other characteristics such as wear resistance and traction. Tires on the market vary in rolling resistance. How these differences in rolling resistance relate to other aspects of tire operating performance and cost is an empirical question that can be addressed by examining tires that are available and in common use today.
Data on rolling resistance characteristics for large samples of passenger tires proved scarce. Measurements from only a few hundred tires have been reported publicly since the mass introduction of radial-ply tires more than three decades ago. These data, derived from varied sources such as the U.S. Environmental Protection Agency and Consumer Reports magazine, are reported to the extent possible, but some are not analyzed any further because of uncertainties and limitations in measurement and
4 Presentations and a summary of the IEA conference can be found at www.iea.org.
1 4 Tires and Passenger Vehicle Fuel Economy
sampling methods. Some of the data sets contain additional information on tire characteristics such as tread wear, traction, and price, but most do not.
The largest and most current set of data containing measurements of tire rolling resistance was made available by three tire manufacturers during the course of the study. These data are analyzed statistically in this report, although the results are accompanied by a number of caveats concerning their relevance to the full population of tires on the replacement market. The majority of the data came from one tire manufacturer; hence, the degree to which the data are representative of tires on the market is not established. The rolling resistance values reported were derived from tests performed on single tire specimens for each tire model and size. Ideally, more tires would have been tested from each tire model to enhance measurement accuracy and ensure the absence of anomalous results. Standardized rolling resistance measurement methods were used, but variations in testing machinery could have affected the comparability of the data reported by different tire companies. Although the sampling was not scientific and the method of data collection was not fully satisfactory, the committee believes that the tire company data, when properly characterized and coupled with information from other replacement tire samples and information obtained by the committee on OE tires, provide useful insights into the rolling resistance and other characteristics of new passenger tires.5
With this information in hand, the committee sought to address the questions asked in the study charge. However, the data provided by tire manufacturers were not made available to the committee until late in the study, which limited the statistical analyses that could be performed. The analyses that were performed are intended to uncover general patterns. Some elements of the questions asked by Congress required interpretation and clarification by the committee—for example, in determining what constitutes “technically feasible” and what is meant by the “average American drive cycle.” One could maintain that only those
5 The State of California is sponsoring the testing of approximately 120 passenger tires for rolling
resistance. It is also testing a portion of the sampled tires for other characteristics such as wet traction and wear resistance. The test results, expected to be available in August 2006, may shed additional light on the issues examined in this study.
Introduction 1 5
tires already for sale are demonstrably “feasible” from both a technical and economic standpoint. Still, technologies throughout the development process can be assessed for technical and economic feasibility. With regard to the “average American drive cycle,” there are many different types of drive cycles. Distilling all U.S. driving activity into a single representative cycle would be a formidable task. Among the many complicating factors are the variability in trip durations and speeds; vehicle types and applications; ambient temperatures, rain, and snow; tire inflation pressures and loads; and road surface types, textures, and temperatures. The committee decided that the most appropriate “average American drive cycle” is simply total miles traveled divided by total fuel consumed by passenger vehicles, since energy expended on rolling resistance is more a function of miles traveled than travel speed.
The meaning of tire “performance” also required some interpretation. An examination of all aspects of tire performance would risk becoming a wide-ranging assessment of all potential relationships between rolling resistance and the multitude of tire qualities that are of interest to motorists, such as noise, handling, appearance, speed capability, and ride comfort, as well as traction and wear resistance. The committee could not think of a meaningful way to assess all possible effects. The dimensions of tire performance specifically mentioned in the congressional charge are energy (fuel), safety, and wear performance. Accordingly, the committee chose to focus the study on those three aspects of performance, with traction deemed to be the characteristic most relevant to assessing effects on safety performance.
The study did not examine all societal effects associated with improving tire energy performance. The focus is limited to direct effects on the consumer. The consumer in this case is the U.S. motorist. Congress asked for estimates of the effects of low-rolling-resistance replacement tires on consumer expenditures for tires and fuel. Society as a whole is also affected by changes in the rate of scrap tire generation and motor fuel consumption, as well as the energy and materials used in tire production. Tracing through and quantifying these broader societal effects, however, would require consideration of outcomes ranging from local air pollution to greenhouse gas buildup. While such a broader accounting of effects may be relevant to policy making, it is beyond the scope and capabilities of this study.
1 6 Tires and Passenger Vehicle Fuel Economy
REPORT ORGANIZATION
Chapter 2 provides context and background on the passenger tire’s development, use, and regulation. Chapter 3 examines tire rolling resistance and its effect on motor vehicle fuel economy. It examines the sources of rolling resistance, methods for testing and measuring rolling resistance, and the range and variability in rolling resistance among new passenger tires. The effects of incremental changes in rolling resistance on motor vehicle fuel economy and consumption are also calculated. Chapter 4 examines relationships among rolling resistance, tire wear life, and traction, including the latter’s bearing on motor vehicle safety. Chapter 5 examines and estimates the effects of lower rolling resistance on consumer expenditures on fuel and tires. The study’s key findings, conclusions, and recommendations are presented in Chapter 6.
REFERENCE
Davis, S. C., and S. W. Diegel. 2004. Transportation Energy Data Book: Edition 24. Report
ORNL-6973. Center for Transportation Analysis, Oak Ridge National Laboratory, Oak Ridge, Tenn.
2
Background on Passenger Tires
This chapter begins with an introduction and overview of basic terminology and trends pertaining to passenger tires and their use in the United States. The introductory discussion is followed by background on the development of tires, the structure of the tire industry, and tire regulations and standards.
TIRE TERMINOLOGY AND TRENDS
Pneumatic, or air-filled, tires are used on vehicles as diverse in form and function as airplanes, bicycles, tractors, and race cars. Accordingly, they encompass a wide range of sizes, designs, materials, and construction types. Nevertheless, structural elements that are common to all of these tires are the casing, bead, and tread band.
The casing—often called the carcass—is the structural frame of the tire. It usually consists of directionally oriented cords banded together by rubber into layers, called plies, which give the tire strength and stiffness while retaining flexibility. The number of plies is determined by tire type, size, inflation pressure, and intended application. Plies oriented mainly from side to side are “radial,” while plies oriented diagonally are “bias.” In the area where the tread is applied, the plies in the radial casing are usually covered by a relatively stiff steel belt or a steel belt covered by a circumferential nylon cap ply. The steel belt is made by using fine wire twisted into cables as cords. For the inflated tire to be retained on the wheel rim, the plies are anchored around circumferential hoops made of multiple strands of fine, high-tensile wire located at the inner edges of the two sidewalls where they mate with the rim. These two hoops, called beads, are pressed against the rim flange by inflation pressure,
1 8 Tires and Passenger Vehicle Fuel Economy
thereby seating and sealing the tire on the rim. Encircling the tire is the tread. This is a thick band of rubber that forms the tire surface, from its crown (its largest radius) to its shoulders (the areas in which the tread transitions to the sidewalls).
The tread is the only part of the tire that comes in contact with the road surface during normal driving. The tread band consists of a grooved section on top of a base. The tread’s design, including its grooved pattern, helps in the removal of road surface water and other contaminants from under the tire while maintaining an adequate level of frictional adhesion between the tire and road to generate torque, cornering, and braking forces under a wide range of operating conditions. For most passenger tires, the grooves start out 9/32 to 13/32 inch deep. Tires are normally considered worn when only 2/32 inch of tread remains.
Most steel-belted radial passenger tires weigh more than 20 pounds, and they can exceed 50 pounds. The steel typically makes up about 15 percent of the total weight, the cord material another 5 percent, and the rubber compound in the carcass and tread about 80 percent (Modern Tire Dealer 2006, 51). Most of the rubber compound’s weight is from natural and synthetic polymers and reinforcing fillers. Other materials added to the compound during processing, such as oils, can contribute 3 to 25 percent of its weight. Because these compounding materials can account for about half of a tire’s total production cost, fluctuations in material prices can have important effects on tire retail prices (Modern Tire Dealer 2006, 46).
The largest application of pneumatic tires is on highway vehicles, which consist of heavy and medium trucks, commercial light trucks, and cars and light trucks used as passenger vehicles. Heavy and medium trucks range from buses to tractor-trailers and construction vehicles. Their tires are designed for heavy workloads, long-distance travel, and rough terrain. Commercial light trucks include many full-size pickups and vans, as well as some SUVs. Their tires are designed mainly for rough terrain and heavy loads. Cars and light-duty trucks used for passenger transportation are the most common vehicles on the highway. Their tires are designed mainly for ride comfort, traction, handling, and wear life, as well as appearance and affordability.
The focus of this study is on tires used on passenger cars and lightduty trucks. The federal government defines and regulates these passen
Background on Passenger Tires 1 9
ger tires in the Federal Motor Vehicle Safety Standards (FMVSS), which are described later in this chapter. All cars are equipped with passenger tires, which usually contain the prefix “P” before their metric size designation molded into the tire sidewall. Even though they are classified as light trucks by the federal government, most SUVs, pickups, and vans used as passenger vehicles are equipped with passenger tires. The kinds of light- and medium-duty trucks used in commercial service, including full-size pickups and vans, have a gross vehicle weight rating of more than 6,000 pounds. These vehicles are usually equipped with tires having the letters “LT” molded into the sidewall. Designed for heavy loads and rough terrain, the LT tires are regulated separately by the federal government and are not part of this study. As a practical matter, the focus is on P-metric tires.
Passenger tires are supplied to automobile manufacturers as original equipment (OE) and to motorists in the replacement market. Statistics on annual shipments of passenger tires for both OE and replacement uses are shown in Figure 2-1. More than 250 million passenger tires were
300
250 200 150
100
50
0
1990 1992 1994 1996 1998 2000 2002 2004
Year
FIGURE 2-1 Passenger tire shipments in the United States replacement and OE markets, 1990-2004. (SOURCE: RMA 2005, 11-12.)
2 0 Tires and Passenger Vehicle Fuel Economy
shipped in the United States in 2004, including about 199 million replacement tires and 53 million OE tires.1 Thus, replacement tires account for
about 80 percent of passenger tire shipments. According to tire dealer data, Americans spent about $20 billion on replacement passenger tires in 2005 (Modern Tire Dealer 2006, 42).
Tire shipment statistics reflect the changing size, age composition, and patterns of use of the U.S. motor vehicle fleet. The number of passenger vehicles in the fleet rose by 21 percent from 1990 to 2002. It was boosted by the addition of 14 million to 17 million new vehicles sold each year and a tendency for vehicles to remain in service longer (Davis and Diegel 2004, 4-5, 4-6). Passenger vehicles are driven an average of 12,000 miles per year, which is an increase of nearly 10 percent since 1990 (Davis and Diegel 2004, 4-2, 4-3). The combination of a growing fleet, vehicles lasting longer, and vehicles being driven more miles has fostered growth in the tire replacement market, which experienced a 33 percent increase in shipments from 1990 to 2004.
HISTORY OF TIRE DEVELOPMENT
The history of passenger tire development is punctuated by innovations and improvements in tire designs, materials, and manufacturing techniques. Three major periods of development merit attention: (a) the
early era coinciding with the mass introduction of the automobile from the early 1900s into the 1930s; (b) the middle of the 20th century, when synthetic rubber became common and major design innovations such as tubeless and radial-ply tires came about; and (c) the period since the mass introduction of radial tires in North America beginning in the 1970s.2
1 Data on tire shipments are provided by the Rubber Manufacturers Association (RMA) and do not
include shipments by companies that are not members of the association. RMA estimates that 79 million tires were imported in 2004 and that 68 million of them were manufactured by RMA companies (RMA 2005, 18). This differential suggests that about 11 million tires were imported by companies that are not members of RMA. Presumably, most of these 11 million tires were sold in the replacement market. The 11 million are not reflected in Figure 2-1.
2 Historical information in this section was derived from the following sources: T. French 1989; Tomkins 1981; RMA 2005; M. French 1989; Rajan et al. 1997; Lindemuth 2005; and Moran 2001.
Background on Passenger Tires 2 1
Early Tire Developments
In the 1840s, Charles Goodyear invented the rubber mixing and curing process known as vulcanization, which was critical in making natural rubber a useful material for a wide range of products. John Boyd Dunlop patented the pneumatic tire for use on bicycles in the 1880s, and by the end of the century, Michelin in France, Goodrich in the United States, and others had adapted the pneumatic tire to the automobile. Within a few years, many companies with now familiar brand names were making tires, including B. F. Goodrich, Firestone, General, Goodyear, and U.S. Rubber (later Uniroyal) in the United States and Continental, Dunlop, Michelin, and Pirelli in Europe.
By World War I, tens of thousands of cars, trucks, and buses were being mass produced each year in the United States, which created a burgeoning demand for tires and many other rubber products such as hoses, belts, and gaskets. New mixers, conveyor systems, and other time- and labor-saving equipment enabled tire production to keep pace with the growing output of automotive assembly lines. Nevertheless, the rapid changes in automobile technologies, new road surfaces, and faster and more frequent driving created new performance demands on tires. In this fast-changing environment, tire companies were forced to learn much about tire design and construction.
Seeking a competitive advantage, tire companies began to invest more in research and development. They found that by replacing the rubbercoated and cross-woven canvas in the tire’s casing with plies of rubberized and directionally oriented fabric, the tire’s fatigue life was greatly extended. They also found that adding reinforcing agents, such as carbon black powder, to natural rubber greatly increased its resistance to abrasion and allowed tires to operate thousands of miles, rather than hundreds, before wearing out. The discovery of many other valuable rubber additives followed and further extended tire service life by slowing degradation from oxygen, heat, ultraviolet radiation, ozone, and moisture.
The gains in tire wear life were accompanied by gains in operational performance, as understanding grew about the tire’s central role in vehicle steering, handling, and braking. Aided by improvements in tire molds and rubber compounding, tire makers introduced better gripping and more durable tread patterns during this period. The bias-ply construction,
2 2 Tires and Passenger Vehicle Fuel Economy
in which plies are oriented diagonally and at alternating angles, became common. This construction, along with the introduction of the steel rim, allowed the tire to support more weight—and thus enabled cars to become larger and heavier during the 1920s and 1930s.
Midcentury Developments
When Japan gained control of Asian rubber plantations during World War II, the United States imposed strict controls on rubber consumption by sharply curtailing the production of tires for nonmilitary purposes and by rationing motor fuel and thus driving activity. At the beginning of the war, the federal government estimated that rubber production could be sustained to meet wartime needs for only about 3 years; hence, it called on the nation’s chemical companies and research institutions to accelerate the development and introduction of synthetic rubbers made from petroleum and natural gas. This major research and development effort was highly successful and resulted in the annual production of hundreds of thousands of tons of synthetic rubber by 1944.3
Having gained experience with synthetics on military tires, tire companies adapted them to passenger tires after the war. When used in tread, synthetic rubber was found to have elasticity characteristics helpful in improving traction. Impermeable synthetic rubbers could be molded into tire inner liners, which allowed the development of tubeless tires. They improved tire puncture resistance by retaining air when damaged and were much easier to mount. By the 1950s, more than two-thirds of the rubber used in tires was synthetic (RMA 2005, 10). Another important development in tire technology in the decade after World War II was the advent of the steel-belted radial-ply tire and its commercial introduction in Europe by Michelin. Radial-ply tires differed in several respects from bias-ply tires. Whereas the cords in biasply tires run diagonally, the carcass cords in radial-ply tires run more directly from bead to bead, perpendicular to the tire’s circumference— an orientation made possible because the tread is stabilized by a stiff cir
3 A history of this period of the tire industry’s development is given by Morawetz (2002) and is re
counted in the video Modern Marvels—Rubber aired by the History Channel and available at www.historychannel.com.
Background on Passenger Tires 2 3
cumferential belt. Today, the belt plies are usually reinforced by small cords made of fine steel cable.
The radial-ply tire offered two critical advantages: a much more stable tread foundation and a more flexible sidewall. These advantages translated into the practical outcomes of longer tread life, better wet and dry traction, improved puncture resistance, and reduced rolling resistance and energy consumption.
Modern Radial Era
As American motorists began driving foreign vehicles and some U.S. models equipped with radial-ply tires during the 1970s, they began demanding these tires in larger numbers. By the beginning of the 1980s, radial tires had become the standard construction type for both OE and replacement tires. Radials accounted for about 60 percent of passenger tire shipments in 1980, 97 percent by the end of the 1980s, and 99 percent in 2005 (Modern Tire Dealer 2006, 51).
Tire wear life was a key selling point for radials, because average tire wear life increased by thousands of miles. In addition, tire companies marketed “all-season” tires made possible by the stability of the steel belt as a structural foundation, which prevented tread cracking in the required cross-groove pattern for winter traction. This development brought an end to the practice among many North American motorists of switching to specialized snow tires during the winter months.
Radials also offered improved handling, which led to a growing array of tires designed and marketed as “performance,” “high performance,” and “ultra-high performance.” Starting in the 1980s, tire manufacturers started rating more tires in North America according to their designed maximum operating speed. The desired speed rating affected the choice of materials and construction of the tire. For instance, tires with higher speed ratings required stronger steel belts and belt compounds covered by a nylon cap ply. The speed rating letter is printed on the passenger tire’s sidewall after sizing information.4 The most common speed rating
4 The rating is based on laboratory tests during which the tire is pressed against a 1.7-meter-diameter
metal drum to reflect its appropriate load and is run at ever-increasing speeds (in 6.2-mph steps in 10-minute increments) until the tire’s rated speed is met.
2 4 Tires and Passenger Vehicle Fuel Economy
TABLE 2-1 Common Speed Ratings for U.S. Passenger Tires
Percentage of Percentage of
Speed Total OE Tire Total Replacement
Rating Speed Speed Shipments Tire Shipments
Symbol (mph) (km/h) Example Applications in 2004 in 2004
S, T 112-118 180-190 Family sedans and vans 83 74
H, V 130-149 210-240 Sport sedans and coupes 15 22
W, Y, Z >149 >240 High-performance sports cars 2 4
SOURCE: RMA 2005, 22.
symbols, maximum speeds, and typical applications for U.S. passenger tires are shown in Table 2-1.
While tire manufacturers do not recommend driving at the top speeds for each speed-rated tire, they use the ratings as one means of distinguishing tires with different performance capabilities. In general, tires rated for higher speeds will also be designed to offer superior performance in a number of respects other than speed, such as handling and steering response. The ratings help motorists maintain vehicle speed capability when they replace speed-rated OE tires.
Figure 2-2 displays the information molded in the passenger tire sidewall, including the size designation that usually follows the tire’s
FIGURE 2-2 Passenger tire sidewall information and major dimensions. (SOURCE: www.tireguides.com.)
Background on Passenger Tires 2 5
name. The tire’s section width (in millimeters) is the first number in the size designation, followed by its aspect ratio, which is calculated by dividing the tire’s section height by its section width and multiplying by 100. Rim diameter (in inches) is the last number in the series, after “R” for radial. Hence a passenger tire with size designation P215/65/R15 has a section width of 215 millimeters, an aspect ratio (or profile series) of 65, and an inner circumference to fit a rim 15 inches in diameter.
Tire industry survey data indicate that eight of the 10 most popular OE tire sizes for Model Year 2005 passenger vehicles fit 16- and 17-inch rims. Because it takes 3 or more years for OE sizing trends to make their way to the replacement market, tires with 15-inch rim sizes remained common among replacement tires in 2005 (Table 2-2). The OE data in Table 2-2 show the growing popularity of tires with larger section widths and lower aspect ratios—trends that have also become more evident in the replacement market with the availability of “plus-size” custom wheels to replace the original wheel and tire combination.
With regard to possible future trends in the replacement market, tires with specially reinforced sidewalls, known as run-flat tires, have grown in popularity in the OE segment. Although they accounted for less than
TABLE 2-2 Passenger Tire Size Popularity, 2005
Percentage of Percentage of
Total OE Tires Replacement Total Replacement
OE Tire Size Shipped Tire Size Tires Shipped
P215/60/R16 6.0 P232/60/R16 6.4
P205/65/R15 5.2 P235/75/R15 6.0
P265/70/R17 5.0 P205/65/R15 4.7
P245/65/R17 4.6 P215/70/R15 4.0
P235/70/R16 4.3 P205/70/R15 3.7
P195/60/R15 3.5 P195/65/R15 3.4
P245/70/R17 3.2 P185/65/R14 3.1
P205/60/R16 3.0 P195/60/R15 2.7
P225/60/R17 2.8 P195/70/R14 2.7
P265/65/R17 2.6 P205/55/R16 2.4
Total, top 10 40.2 Total, top 10 39.1
SOURCE: Modern Tire Dealer 2006, 45.
2 6 Tires and Passenger Vehicle Fuel Economy
1 percent of replacement sales in 2005, their rate of growth will be influenced by OE acceptance (Modern Tire Dealer 2006, 46). These air-filled but partially structure-supporting tires are designed to operate with the loss of inflation, down to zero inflation pressure for speeds up to 55 mph for a distance of up to 50 miles. Originally developed for two-seat sports cars with little room for spare tires and jacks, run-flat tires can now be found on other passenger vehicles. They are marketed for their convenience and safety in the event of a flat in a remote or hazardous location. As noted later in the report, run-flat tires weigh more than conventional radial tires—which increases their material and production cost—and they tend to exhibit higher rolling resistance.
TIRE INDUSTRY STRUCTURE
The tire industry is international and driven by competition. The majority of OE and replacement tires sold in the United States are produced by several large domestic and foreign manufacturers, all operating internationally, including Michelin (France), Goodyear (United States), Bridgestone/Firestone (Japan), Pirelli (Italy), Cooper (United States), Toyo (Japan), Kumho (South Korea), Continental (Germany), Hankook (South Korea), Yokohama (Japan), and Sumitomo (Japan). Potentially adding to the competitive mix in the replacement market is the growing number of passenger tires produced by companies based in China, Taiwan, India, and other industrializing countries (Modern Tire Dealer 2006, 51).
Tire manufacturers supply the two distinct—albeit related—markets: OE and replacement. Automobile manufacturers buy in large volumes that give them influence over tire prices and specifications. They demand tires with characteristics that suit their vehicle designs, marketing strategies, and production schedules. In turn, OE orders allow tire companies to keep their production facilities operating at efficient volumes. The OE business also can help generate future sales of replacement tires. By linking its tire lines with a specific vehicle make or model, a tire company can draw on the brand loyalty of motorists. Because four times as many replacement tires as OE tires are sold, such brand loyalty can be valuable to the tire manufacturer.
Background on Passenger Tires 2 7
Like makers of many other consumer goods, tire manufacturers seek to distinguish their products from those of competitors through branding. Most sell under heavily advertised manufacturer (or national “flag”) brands as well as associate and specialty brands, some acquired through mergers and acquisitions of well-known tire companies. Goodyear, for instance, sells under its own name and several other nationally recognized brands; it owns Dunlop (in the United States) and Kelly. Likewise, Michelin has acquired the BFGoodrich and Uniroyal brands in the United States, and Bridgestone also sells tires under the Firestone and Dayton brand names. These nine brands accounted for 51.6 percent of the replacement tire consumer market in 2005 (Modern Tire Dealer 2006, 39).
Most major tire companies supply both the OE and the replacement markets. They typically use their flag brands for the former and a combination of flag and associate brands for the latter. An exception to this practice is Cooper Tire, which concentrates on serving the replacement market. It sells tires under its own brand name and under associate brands such as Starfire, Dean, and Mastercraft. In addition, most tire makers supply replacement tires to retailers selling under private labels, such as the Sears Guardsman, Wal-Mart Douglas, and Pep Boys Futura. In these cases, the retailer creates and controls the brand, often contracting for supplies from one or more tire makers offering the lowest price or other valued attributes such as supply reliability.
OE Market
OE tires outfitted on a specific vehicle are usually developed and supplied by one or two preselected tire makers. From the standpoint of the automobile manufacturer, it can sometimes be advantageous to engage at least two OE tire suppliers to ensure an ample and timely supply and to foster competition. As part of the development process, experimental tires are usually submitted to the automobile manufacturer by the tire maker, along with various test measurements. The tires are evaluated, and further refinements are made as needed. Most automobile companies have in-house tire testing facilities and expertise to assist in tire evaluation and specification.
OE tires are usually specified in both quantitative and qualitative terms. The OE specification sheet will define the tire’s physical dimensions, such as mass, width, and diameter within the parameters of tire
2 8 Tires and Passenger Vehicle Fuel Economy
and rim standards. Because the tire is integral to the vehicle’s suspension, steering, acceleration, and braking, the automobile maker will also set precise and quantifiable targets for properties such as force and moment (cornering coefficient, aligning torque coefficient, etc.); deflection (spring rate); and traction (friction coefficients) in wet, dry, and snow conditions. Other quantifiable properties that are usually specified include electrical conductivity (resistance to static shock), speed endurance (suitable to the vehicle’s speed capability), tire wear resistance, and rolling resistance (rolling resistance coefficient).5 In addition, the auto
mobile manufacturer will define several other tire attributes, sometimes through more qualitative means, such as the tire’s expected noise and vibration levels, sidewall appearance, and tread image.
Some OE tire specifications are governed by FMVSS such as those covering tire structural safety and rim selection. These apply to all passenger tires. Other OE specifications are strongly influenced by the federal safety standards and other regulations applying to motor vehicles. For example, OE tire designs are influenced by federal standards for passenger vehicle brake systems and motor vehicle fuel economy.
Replacement Market
The logistics of tire manufacturing, inventorying, and distribution in the replacement market are focused on serving the complete market. Most replacement tires are designed to perform on the wide range of vehicles in the fleet, including vehicle models dating back many years. Hence, whereas the OE market is characterized by the supply of large quantities of select tire types and sizes, suppliers competing in the replacement market must offer a wide variety of tire sizes and types, generally produced in smaller quantities. As a result of market competition, evolving consumer demands and preferences, and changing tire dimensions and specifications introduced in the OE segment, the spectrum of replacement tire sizes and types is continually expanding.
At any one time, replacement tires from hundreds of brands and lines are for sale in the marketplace, which consists of tens of thousands of individual products, or stock-keeping units, when size variability is taken
5 See Lindemuth (2005) for a more detailed listing of performance criteria and measures.
Background on Passenger Tires 2 9
into account. Consumers may choose among a handful to several dozen tire lines for their replacement needs. The choices range from national Internet and mail-order companies to tire dealers, manufacturer outlets, and retail department stores (Figure 2-3). Typically, the tires bought in the replacement market are balanced and mounted by the tire dealer, who adds about $50 to the cost of purchasing a set of four tires (Modern Tire Dealer 2006, 55).
TIRE SAFETY AND CONSUMER INFORMATION STANDARDS
Even as they market their products to differentiate among tire brands and lines, tire companies recognize the value of standardization. Early
FIGURE 2-3 Distribution channels for replacement tires in the United States. (SOURCE: RMA 2005, 13.)
3 0 Tires and Passenger Vehicle Fuel Economy
in its history, the tire industry suffered from excessive product differentiation, especially in tire dimensions. Tires designed and configured for just one vehicle proved costly and difficult to replace when damaged or worn. Automobile manufacturers therefore advocated common size designations to promote interchangeability and competition in supply. Today’s passenger tires must conform to a number of standards. Some are required by government, while others are adopted voluntarily by industry and developed through national and international standardsetting bodies. Tire speed ratings, as previously discussed, are an example of a standard developed and implemented by industry. The following subsections describe those standards for passenger tire safety and consumer information that are required by the federal government.6
Federal Safety Regulations for Passenger Tires
Between 1966 and 1970, Congress passed several acts defining and expanding the federal government’s role in regulating motor vehicle safety and creating the National Highway Traffic Safety Administration (NHTSA) under the U.S. Department of Transportation to implement them. NHTSA promulgated a series of FMVSS affecting various systems and components of the motor vehicle, such as interior displays and controls, brakes, and occupant protection devices. The rules governing tires cover two main areas: tire structural integrity and fitment.
With regard to structural integrity, the regulations prescribe a battery of tests that must be passed demonstrating
• Tread plunger strength (a round hub is pressed against the tread with
a given force to test strength),
• Resistance to bead unseating,
• High-speed performance at constant load and variable speed, and • Endurance at constant speed and variable load.
After passage of the federal TREAD Act of 2000,7 a low-pressure tire endurance test was developed for introduction, along with additional
6 See Walter (2005) for a more detailed review of government and industry standards and regula-
tions pertaining to passenger tires.
7 The Transportation Recall, Enhancement, Accountability, and Documentation (TREAD) Act
(Public Law 106-414) was signed into law on November 1, 2000.
Background on Passenger Tires 3 1
requirements for the testing of tire endurance. These requirements are scheduled to take effect in 2007. More additions to the regulations are anticipated in response to the TREAD Act as NHTSA examines tests for tire aging.
With regard to tire sizing and fitment, the federal regulations require that all tires conform to standards for size, load, and pressure relationships developed by standard-setting bodies such as the U.S. Tire and Rim Association, the European Tire and Rim Technical Organization, and the Japan Automobile Tire Manufacturers Association.8 NHTSA requires tire makers to print sizing information on the tire sidewalls. Tires in compliance with the federal safety standards are marked with the “DOT” symbol (for U.S. Department of Transportation), along with additional information such as the location and date of tire production, maximum pressure, and tire material and construction type.
Other FMVSS regulations influence tire design and construction, including braking standards for motor vehicles. Recently, NHTSA adopted a new rule that will require tire pressure monitoring systems to be installed on all new passenger cars and light trucks starting with 2007 vehicle models.
Federal Consumer Information Requirements for Passenger Tires
Separate from the federal tire safety requirements are federal requirements intended to provide consumers with information for making tire purchases. The Uniform Tire Quality Grading (UTQG) system applies to all passenger tires with the exception of winter tires and compact spares. In its current form since 1980, the UTQG system consists of grades for tread wear, wet traction, and temperature resistance. Manufacturers typically test one or more tire models from a tire line or grouping to establish the grades for each of the three qualities, which are then molded on the tire sidewall.
8 Other bodies include the Deutsche Industrie Norm, the British Standards Institution, the Scan
dinavian Tire and Rim Organization, and the Tyre and Rim Association of Australia.
3 2 Tires and Passenger Vehicle Fuel Economy
Tread Wear Grade
UTQG tread wear grade is a comparative rating generated from the results of an outdoor highway test course in which the subject tire is run in a convoy with several standardized “course-monitoring” tires. After 7,200 miles, the subject tire’s wear rate is compared with that of the monitoring tires. The tire manufacturer assigns a tread wear grade on the basis of extrapolations of measured wear rates. The ranking scheme suggests that a tire rated 200 should wear twice as long as a tire rated 100 on the government test course. The relative performance of tires, however, depends on the conditions of use, and therefore it may depart significantly from the norm because of variations in operating conditions and maintenance. The 2,371 rated passenger tire lines have the following distribution of tread wear grades according to information on NHTSA’s website:9 200
or lower, 11 percent; 201 to 300, 21 percent; 301 to 400, 33 percent; 401 to 500, 22 percent; 501 to 600, 8 percent; above 600, 5 percent. Neither NHTSA nor tire manufacturers are willing to associate expected mileage levels with particular grades because of the variability in wear that can occur on the basis of vehicle operating conditions, road conditions, tire maintenance, and individual driving patterns.
Traction Grade
UTQG traction grades are based on a tire’s measured coefficient of friction when it is tested on wet asphalt and concrete surfaces. The subject tire is placed on an instrumented axle of a skid trailer, which is pulled behind a truck at 50 mph on wet asphalt and concrete surfaces. The trailer’s brakes are momentarily locked, and sensors on the axle measure the longitudinal braking forces as it slides in a straight line. The coefficient of friction is then determined as the ratio of this sliding forced to the tire load. Grades of AA, A, B, and C are assigned according to the criteria shown in Table 2-3.
Traction grades are intended to indicate a tire’s ability to stop on wet pavement. The UTQG traction grade does not take into account other aspects of traction, such as peak traction, traction on dry or snow-covered surfaces, or cornering traction. NHTSA website data indicate that of the
9 www.safercar.gov/tires/pages/Tires2.cfm. Results reported to NHTSA are not sales weighted.
Background on Passenger Tires 3 3
TABLE 2-3 UTQG Traction Grades
Traction Wet Asphalt Sliding Wet Concrete Sliding
Grade Friction Coefficient Friction Coefficient
AA >0.54 >0.38
A >0.47 >0.35
B >0.38 >0.26
C <0.38 rrc =" 0.008)" id="5973.">30
Goodyear 4
Speed rating
S, T 97 59.9
H, V 31 19.1
W, Y, Z 34 21.0
Rim size (in.)
13 5 3.1
14 18 11.1
15 47 29.0
16 43 26.5
17 30 18.5
18 10 6.2
19+ 9 5.6
Tread depth (where known)a (in.)
9/32 7 5.1
10/32 58 42.0
10.5/32 6 4.3
11/32 40 29.0
11.5/32 2 1.4
12/32 8 5.8
13/32 or more 17 12.3
Tire weight (where known)b (lb)
<20 sample =" 154;" sample =" 8;" tires =" 162." depth =" 10.76/32" weight =" 26.6">0.54 >0.38 4 21 42
A >0.47 >0.35 78 72 141
B >0.38 >0.26 18 7 13
C <0.38 observations =" 170;" r2 =" .52;" r2 =" .50." observations =" 164;" r2 =" .51;" r2 =" .50.">0.008 to 0.009 >0.009 to 0.01 >0.01 to 0.011 >0.011
14-inch S, T
Number of tires 1 1 7 10 6
Average RRC 0.0061 0.0088 0.0097 0.0107 0.0117
Average price ($) 71.00 48.00 59.00 65.70 59.30
15-inch S, T
Number of tires 0 6 14 12 6
Average RRC NA 0.0085 0.0097 0.0105 0.0117
Average price ($) NA 70.33 75.57 79.41 71.80
16-inch S, T
Number of tires 2 4 13 5 4
Average RRC 0.0067 0.0087 0.0944 0.0104 0.0114
Average price ($) 93.50 102.00 104.00 102.20 85.25
16-inch H, V
Number of tires 0 2 7 4 3
Average RRC NA 0.0085 0.0093 0.0105 0.0117
Average price ($) NA 113.50 147.00 113.25 86.00
NOTE: NA = not applicable. RRC values were measured when tires were new.
Tire Wear Effects
The findings in Chapter 4 suggest that new-tire rolling resistance can be reduced by a magnitude of 10 percent by reducing tread depth by about 22 percent. At the same time, the data suggest that tires with reduced tread depth exhibit shorter wear life. Indeed, lower Uniform Tire Quality Grading tread wear numerical ratings—by about 5 percent—were observed for each 1/32-inch reduction in tread depth. This is equal to about 9 percent of tread depth for the average tire. If consumers were to purchase more tires with less tread as the main way to achieve lower rolling resistance, they would likely experience shorter wear life and need to replace their tires more often.
Perhaps the simplest way to approximate the effects of shorter wear life on tire replacement expenditures is to use the figures in Chapter 2 indicating that about 200 million replacement tires are shipped in a year
National Consumer Savings and Costs 1 1 3
for use on 175 million passenger vehicles. The ratio of vehicles to tires (175 million/200 million = 0.88) suggests that a motorist can expect to
purchase a replacement tire an average of every 0.88 year, or a complete set of four tires about every 3.5 years (4 × 0.88 = 3.52).7 If reductions in
rolling resistance are brought about by consumers purchasing tires with thinner tread, the frequency of tire purchases would increase by an amount commensurate with the reduction in tire wear life.
Suppose that the average tread depth of new tires purchased decreases by 22 percent. The analyses in Chapter 4 suggest that such a change would reduce new-tire RRCs by about 10 percent and projected wear life by about 10 percent. Accordingly, the number of replacement tires purchased in a year would need to increase by about 10 percent, from 200 million to about 220 million. Motorists would thus purchase a new tire on average every 0.80 year (175 million/220 million), or a complete set of four tires every 3.2 years. In terms of annual tire expenditures, the motorist would purchase an average of 1.25 tires per year (4/3.2), as opposed to the current average of 1.14 tires per year (4/3.5).
The full cost to the consumer of having to buy an average of 0.11 more tires per year will depend on tire prices and other tire transaction and installation costs. The average price of tires in the combined Ecos Consulting and RMA data is $117. The data set, however, contains a large number of high-performance tires. While tires rated for higher speed (H, V, W, Y, Z) are becoming more popular among U.S. motorists, they do not represent 40 percent of replacement tires sales, which is their percentage in the data set. RMA’s Factbook 2005 indicates that tires rated S and T accounted for 73 percent of replacement tire shipments in 2004, while performance (H, V) and high-performance (W, Y, Z) tires accounted for 22 and 4 percent, respectively (RMA 2005, 22). Weighting the price data by these reported sales percentages suggests an average tire price of $97. Hence, consumer expenditures on tires would increase from an average of $110.58 per year (1.14 × $97) to an average of $121.25 (1.25 × $97) per year, a difference of $10.67.
7 If vehicles are driven an average of 12,000 miles, this figure equates to an average tire life of 42,000
miles (3.5 years × 12,000 miles).
1 1 4 Tires and Passenger Vehicle Fuel Economy
Other costs associated with tire replacement include the expense of installation and the inconvenience and time lost to motorists. These costs are real but difficult to quantify fully. Tire installation (e.g., balancing, mounting, and valve stem replacement) and other associated consumer expenses such as tire disposal fees can vary from $40 to more than $100 for a set of four tires, with $50 (or $12.50 per tire) being the reported average.8
Thus, including these installation costs would add about $1.38 (0.11 × $12.50) to annual tire expenditures, which would bring the total to about $12 more per year ($10.67 + $1.38).
For the 175 million passenger vehicles using replacement tires, the total tire expenditure increase under this scenario would be $2.1 billion per year. In reality, the scenario’s assumption that reduction in tread depth will be the exclusive means of achieving lower rolling resistance is questionable. Tire manufacturers can minimize tread volume and mass by means other than, or in addition to, reducing depth. For instance, tread width, shoulder profile, and section width can be modified to reduce rolling resistance while seeking to minimize adverse effects on wear life. U.S. motorists are known to demand long wear life when they purchase tires, as reflected by the mileage warranties advertised by tire companies. It is improbable that tire manufacturers interested in maintaining customers would sacrifice wear life to any major degree.
In any event, as pointed out earlier, achieving a lower RRC only by reducing tread thickness may not lead to significantly lower rolling resistance on the average over a tire’s lifetime. As it accumulates miles, a tire with thicker tread will soon assume wear and rolling resistance profiles similar to those of an otherwise comparable tire starting out with thinner tread. The fuel savings will occur only during the miles driven before the added tread thickness wears down, if both tires are replaced at the same level of tread wear, and will be limited accordingly. To illustrate with a simplified example, suppose that all tires wear evenly at a rate of 1/32 inch oftreadper 5,000 milesandarereplacedwhentreaddepthreaches 2/32 inch. Further, suppose that RRC declines evenly by 0.005 per 1/32 inch of tread loss, that one tire starts out with a tread depth of 10/32 inch and an RRC
8
Modern Tire Dealer (2006) reports that the average customer expenditure on new-tire mounting and balancing is $49.
National Consumer Savings and Costs 1 1 5
of 0.01, and that another starts out with a tread depth of 12/32 inch and an RRC of 0.011. The former tire’s average RRC over its 40,000-mile lifetime will be 0.00825, while the latter tire’s average RRC over its 50,000-mile lifetime will be 0.00875. In effect, after 10,000 miles of use, the latter tire will assumethesamewearandrollingresistanceprofilesastheformer.Although its RRC starts out 10 percent higher, the latter tire’s lifetime average RRC is only 6 percent higher. The thinner-tread tire will have lower average rolling resistance; however, it will also require replacement 20 percent sooner—not an attractive option from the perspective of consumer tire expenditures or controlling scrap tire populations.
These examples illustrate why reducing rolling resistance by designing tires with less tread depth would have both limited effects on fuel consumption and an undesirable response from motorists—and thus why such an approach would not likely be pursued generally. Indeed, because tire manufacturers must respond to consumer demand for wear resistance, they have sought alternative means of reducing rolling resistance with minimal loss of wear life. Some of these alternatives, including new tread materials, are discussed in the next scenario, along with approximations of their effects on consumer tire expenditures.
Scenario 2: Reducing Rolling Resistance by Changing Tread Composition
Tire manufacturers and their materials suppliers have been actively seeking optimal means of reducing rolling resistance without sacrificing wear life and other aspects of performance. Unfortunately, the study committee is not aware of the various technologies—some proprietary—that have been developed and tried.
However, the important effect on rolling resistance of the tread compound and its constituent rubbers and reinforcing fillers is well established in the literature. Rubbers typically account for between 40 and 50 percent of tread volume and weight, and fillers typically account for 30 to 40 percent (Derham et al. 1988; Bethea et al. 1994; Russell 1993; Gent 2005, 30). Oils and other additives, which are used in processing and as material extenders, account for the rest of the volume and weight.
The tread’s wear resistance, traction, and rolling resistance are determined in large part by the properties of these polymers and fillers, as well
1 1 6 Tires and Passenger Vehicle Fuel Economy
as by their concentrations, dispersion, and adhesion characteristics (Böhm et al. 1995; Wang et al. 2002). Consequently, fillers and polymers, as well as methods for mixing and curing them in the tread compound, have been primary targets of research and development aimed at reducing rolling resistance while preserving acceptable levels of other aspects of tire performance.
As discussed earlier, the predominant filler used in the tread compound is carbon black. A great deal of research has been devoted to modifying carbon black as a means of reducing rolling resistance. Among the approaches investigated have been varying its agglomerated particle size, manipulating its surface structure, and improving its dispersion through reactive mixing and other means (Russell 1993; McNeish and Byers 1997; Wang et al. 2002; Cook 2004). Because the supply of carbon black is a highly competitive business, materials suppliers have devoted much research and development to improving and distinguishing their products with regard to the effects on rolling resistance and other properties.
Silica is the next most common reinforcing filler in the tread compound. Silica has been added to tire rubber for decades, usually in combination with carbon black, largely because it improves cutting and chipping resistance of a tire as well as traction on snow and ice (Derham et al. 1988). However, silica does not develop a natural strong bond with rubber, owing to their different polarities. Silica tends to cluster rather than disperse evenly in the tread compound. This clustering not only makes processing more difficult, it increases the tread’s hysteresis and results in poor wear. In the early 1990s, researchers found that applying organosilane coupling agents to silica during mixing resulted in more uniform filler dispersion and a consequent reduction in rolling resistance. In such applications to achieve lower rolling resistance, the silica-silane usually replaces a portion— seldom more than one-third—of the carbon black in the tread compound. Since this discovery, silica-silane systems have been promoted as a means of reducing rolling resistance without a severe penalty on traction or tread wear.
Replacing or modifying the filler is not the only means of reducing rolling resistance through changes in tread composition. Tread composition can be altered in other ways—for example, through changes in the rubbers, other tread components (e.g., oils, sulfur, zinc), and mixing
National Consumer Savings and Costs 1 1 7
processes. Examples of such modifications include the use of functionalized polymers that foster more uniform filler dispersion. Hydrogenated and tin-modified polymers have been used to reduce the rolling resistance of tires that are in production (Bethea et al. 1994; McNeish and Byers 1997). Of course, a more comprehensive approach to reducing rolling resistance would involve not only modifications of the tread compound but also changes in tire geometry and mass, belt and subtread materials, and the design and construction of other tire components such as the sidewall and casing.
The study committee could not examine all possible means of reducing rolling resistance—even means involving only changes in tread composition. Accordingly, the following estimates focus on the added materialrelated costs associated with a single change in tread composition: the partial substitution of silica-silane for carbon black. This scenario— admittedly simplified—provides an order-of-magnitude estimate of the effects on tire production costs that would be passed along to consumers in the prices paid for replacement tires possessing lower rolling resistance. Market prices for carbon black and silica vary with supply and demand factors, including energy and transportation costs (Crump 2000). The prices paid by tire manufacturers for these materials are usually negotiated with suppliers and are not publicly available. While price differences between carbon black and silica vary at any given time and among suppliers, silica prices tend to be higher than carbon black prices by about one-third. Reference prices are $45 per 100 pounds of carbon black and $60 per 100 pounds of silica. About 5 pounds of silane, which costs about $3 per pound, is used for every 100 pounds of silica. Hence, the silica-silane combination costs about $75 per 100 pounds, compared with $45 for 100 pounds of carbon black. When silica-silane is used to reinforce tread stock, it seldom replaces more than one-third of the carbon black by volume or weight.
For an average passenger tire weighing 26.6 pounds,9 the full tread band accounts for about 25 percent of the weight, or 6.7 pounds. Most of this tread weight is from the polymers as well as oils and other additives used in the tread compound. If it is assumed that reinforcing filler accounts for
9 The average weight of tires in the RMA data set is 26.6 pounds.
1 1 8 Tires and Passenger Vehicle Fuel Economy
35 percent of the tread’s weight, the filler’s total weight is about 2.3 pounds. If carbon black is used exclusively as the filler, its material costs will be $1.04 per tire ($0.45 per pound × 2.3 pounds). Replacing one-third
(or 0.76 pound) of the 2.3 pounds of carbon black with an equal weight of silica-silane will raise the cost of filler material to about $1.26 per tire ($0.45 per pound × 1.54 pounds + $0.75 per pound × 0.76 pound), an increase in filler costs of $0.22 per tire.
Of course, estimates of raw material costs will not capture all manufacturing costs associated with substituting silica-silane for carbon black. The processing of silica-silane differs from that of carbon black. The former usually requires reactive mixing to raise the mixing temperature sufficiently to allow silica and silane to bond. The addition of silane also lengthens the curing time required for tread compounds and produces emissions of ethanol, which is a reactive compound subject to federal and state air quality controls (Joshi 2005). There are reports that silica, which is harder and contains more water than carbon black, can accelerate the wear of mixing devices from abrasion and corrosion (Borzenski 2004). While the added processing time, emissions mitigation, and equipment maintenance may not require large-scale plant investments, they will introduce additional production costs beyond the tread material expenses alone. It is reasonable to assume that these other costs would be at least as large as the silica-silane material expense, which would add another $0.22 to tire production costs and bring the total to $0.44 per tire.
The purpose of these calculations is not to develop a precise estimate of added costs but to get a sense of their scale and potential to translate into higher tire prices. Only the tire manufacturers can offer precise estimates of the effects on production costs and pricing, which are proprietary in nature and will depend in part on fluctuations in material costs and the pricing and cost allocation procedures of individual manufacturers. The estimates, though rough, suggest that the added cost of silica-silane will be less than $0.50 per tire. To be even more cautious, however, the committee assumes a resultant increase of $1 in the retail price of the tire. This added margin factors in the uncertainties noted above with regard to effects on tire manufacturing processes (e.g., emissions mitigation, equipment maintenance) as well as any cost markups that are successfully passed along to consumers. For an average tire priced at $97, a $1
National Consumer Savings and Costs 1 1 9
price increase represents a premium of slightly more than 1 percent and would cause consumer tire expenditures to rise by an average of $1.14 per year assuming that tire wear life remains unchanged (since, on average, 1.14 replacement tires are purchased by motorists each year). For the 175 million passenger vehicles equipped with replacement tires, the total expenditure would be about $200 million per year (175 million × $1.14).
The application of silica-silane would likely be accompanied by other changes in tire materials and designs to achieve lower rolling resistance. Therefore, it is not possible to state with certainty that consumers would only pay about $1 more per tire in practice or that the tires would be comparable in all respects—including wear resistance, traction capability, and other properties—with tires having higher rolling resistance. The calculations do suggest that additional tire production costs are likely to result in a modest, rather than a dramatic, change in tire prices.
Unquestionably, an important consideration for consumers is tire wear life. While silica-silane systems are promoted as having wear and traction characteristics comparable with those of conventional tread compounds, the committee cannot verify these claims. Even a relatively small reduction in average wear life, on the order of a few percentage points, would result in corresponding increases in tire purchases and scrap tires. The estimates presented earlier in this chapter suggest that each 1 percent reduction in tire life would cost motorists an average of about $1.20 more per year in tire-related expenditures. Hence, if average tire life is shortened by as little as 5 percent, all or a significant portion of the annual fuel savings associated with lower rolling resistance would be offset.
OVERALL EFFECT ON CONSUMER EXPENDITURES
The time that might be required to achieve a 10 percent reduction in the average rolling resistance of replacement tires is not considered here because it would depend on the specific means of achieving the reduction. At a minimum, such a reduction would likely require at least as many years as required to turn over most of the tires in the fleet. If new technologies were introduced to bring about the reduction, an unspecified amount of time for product development and market penetration would be required. As calculated above, such a reduction in average rolling
1 2 0 Tires and Passenger Vehicle Fuel Economy
resistance would save motorists an average of $12 to $24 per year in fuel expenditures, or $1.20 to $2.40 for every 1 percent reduction in the average rolling resistance experienced by replacement tires used on passenger vehicles.
Estimating the effect of reducing rolling resistance on tire expenditures is further complicated because of the numerous ways by which rolling resistance can be reduced. To gauge these costs, two scenarios were presented. One assumes that informed consumers would purchase more tires with lower rolling resistance from the selection of replacement tires already on the market. This is a conceivable scenario because today’s replacement tires already exhibit much variation in rolling resistance, even among tires that are comparable in size and various performance ratings. Data available on replacement tires do not show a clear pattern of price differentials among replacement tires that vary in rolling resistance. This suggests that such a shift in consumer purchases would not be accompanied by higher average tire prices and tire expenditures as long as wear resistance does not suffer.
A possible concern is that consumers, demanding fuel economy, would purchase more tires with shorter wear life in the event that reducing tread thickness is the primary means employed by tire manufacturers to achieve lower rolling resistance. The estimates developed here suggest that each 1 percent reduction in tire wear life will cost consumers about $1.20 per year in additional tire expenditures. A shift in purchases that favors tires with shorter wear life could therefore result in higher tire expenditures that offset fuel savings. However, this outcome is unlikely as a practical matter. Not only would the fuel savings from this approach be small, but consumers would quickly observe and seek to avoid the trade-off, given their long-demonstrated interest in prolonging tire wear life. Indeed, reducing tread depth does not appear to be the only, or the most common, method for achieving lower rolling resistance among tires already on the market. Tire manufacturers and their suppliers have been actively researching new materials and technologies to reduce rolling resistance without compromising wear resistance and traction. These materials and technologies tend to be more costly than are those used in conventional tires. Rough estimates of the additional cost of modifying tread composition to reduce rolling resistance suggest a price premium that is on the order of $1 per tire.
National Consumer Savings and Costs 1 2 1
In practice, changes in tread composition to reduce rolling resistance tend to be made as part of more comprehensive changes in tire design, construction, and dimensions. The committee could not find comprehensive quantitative information on how such changes, taken together, would affect tire prices and other aspects of tire performance such as traction and wear resistance.
REFERENCES
Abbreviations
EIA Energy Information Administration
RMA Rubber Manufacturers Association
Bethea, T. W., W. L. Hergenrother, F. J. Clark, and S. S. Sarker. 1994. Techniques to Reduce
Tread Hysteresis. Rubber and Plastics News, Aug. 29.
Böhm, G. A., M. N. Nguyen, and W. M. Cole. 1995. Flocculation of Carbon Black in
Filled Rubber Compounds. Presented to the Society of Rubber Industry, International Rubber Conference, Kobe, Japan.
Borzenski, F. J. 2004. Internal Wear of the Batch Mixer. Presented at International Tire
Exhibition and Conference, Akron, Ohio, Sept. 21-23.
Cook, S. 2004. Low Rolling Resistance and Good Wet Grip with Silica. Tire Technology
International 2004.
Crump, E. L. 2000. Economic Impact Analysis for the Proposed Carbon Black Manufacturing
NESHAP. Report EPA-452/D-00-003. Office of Air Quality Planning Standards, U.S. Environmental Protection Agency, May.
Derham, C. J., R. Newell, and P. M. Swift. 1988. The Use of Silica for Improving Tread
Grip in Winter Tyres. NR Technology, Vol. 19, No. 1, pp. 1-9.
EIA. 2005. Annual Energy Outlook 2006 (early website release). U.S. Department of Energy,
Washington, D.C.
Gent, A. N. 2005. Mechanical Properties of Rubber. In The Pneumatic Tire (J. D. Walter
and A. N. Gent, eds.), National Highway Traffic Safety Administration, Washington, D.C., pp. 28-79.
, P. G. 2005. Low-VOC Silanes. Tire Technology International 2005, pp. 126-129. McNeish, A., and J. Byers. 1997. Low Rolling Resistance Tread Compounds: Some Com
pounding Solutions. Presentation to Rubber Division, American Chemical Society, May 6, Anaheim, Calif.
ModernTireDealer. 2006.ModernTireDealer’sFactsIssue.www.moderntiredealer.com.Jan.
1 2 2 Tires and Passenger Vehicle Fuel Economy
RMA. 2005. Factbook 2005: U.S. Tire Shipment Activity Report for Statistical Year 2004.
Washington, D.C.
Russell, R. M. 1993. Compounding for Wet Grip. Tire Technology International 1993,
pp. 14-19.
Small, K., and K. Van Dender. 2005. Fuel Efficiency and Motor Vehicle Travel: The
Declining Rebound Effect. Economic Working Paper 05-06-03 (revised December). University of California at Irvine.
Wang, M. J., Y. Kutsovsky, P. Zhang, G. Mehos, L. J. Murphy, and K. Mahmud. 2002.
KGK Kautschuk Gummi Kunststoffe 55.
6
Findings, Conclusions, and Recommendations
The technical literature and empirical evidence have been reviewed in this study to gain a better understanding of how the rolling resistance characteristics of tires relate to vehicle fuel economy, tire wear life, traction, and other aspects of tire performance. The focus has been on passenger tires sold for replacement, although it is recognized that original equipment (OE) tires lead many of the design trends and technologies emerging in the replacement market. The study has revealed variability in rolling resistance characteristics among replacement tires. Rolling resistance not only differs among tires when they are new but also changes as tires are used and maintained. The findings in this study make it possible to approximate the effect of a plausible reduction in the average rolling resistance of replacement tires in the passenger vehicle fleet on vehicle fuel economy. They also permit estimation of possible effects on tire wear life and operating performance of means of reducing rolling resistance.
Key study findings and estimates are consolidated to begin the chapter. They provide the basis for a series of conclusions in response to the specific questions asked by Congress. Taken together, the findings and conclusions persuade the committee that consumers will benefit from having greater access to information on the influence of passenger tires on vehicle fuel economy. They will also benefit from complementary information stressing the importance of proper tire inflation and maintenance to fuel economy, safe operation, and prolonged wear. Hence, the committee recommends that the National Highway Traffic Safety Administration (NHTSA) begin gathering this information and communicating it to the public, in close cooperation with the tire industry.
1 2 4 Tires and Passenger Vehicle Fuel Economy
KEY FINDINGS AND ESTIMATES
Rolling resistance has a meaningful effect on vehicle fuel consumption. For conventional passenger vehicles, most of the energy contained in a gallon of motor fuel is lost as heat during engine combustion and from friction in the driveline, axles, and wheel assemblies. Some of the energy produced by the engine is consumed during idling and by vehicle accessories. Only about 12 to 20 percent of the energy originating in the fuel tank is ultimately transmitted to the wheels as mechanical energy to propel the vehicle. Rolling resistance consumes about one-third of this transmitted energy.
In one sense, rolling resistance consumes only a small fraction of the total energy extracted from a gallon of fuel. In another sense, a reduction in rolling resistance will reduce demand for mechanical energy at the axles. This will have a multiplier effect because it will translate into fewer gallons of fuel being pumped to the engine in the first place.
The overall effect of a reduction in rolling resistance on vehicle fuel economy will depend on a number of factors, including the underlying efficiency of the engine and driveline as well as the relative amounts of energy consumed by other factors, such as aerodynamic drag and vehicle accessories. For most passenger vehicles, a 10 percent reduction in rolling resistance will have the practical effect of improving vehicle fuel economy by about 1 to 2 percent.
Tires are the main source of rolling resistance.
The rolling resistance encountered by a vehicle can be extreme when it is driven on a soft or rough surface, such as a gravel or dirt road. On hard paved surfaces, which are more common for the operation of passenger vehicles, the main source of rolling resistance is the repeated flexing of the vehicle’s tires as they roll. Through an effect known as hysteresis, this repeated flexing causes mechanical energy to be converted to heat. More mechanical energy must be supplied by the engine to replace the energy lost as heat from hysteresis. The design, construction, and materials of tires, as well as their maintenance, their condition, and operating conditions, affect the rate of energy loss. For most normal driving, a tire’s rolling resistance characteristics will not change in response to an increase or decrease in vehicle travel speed.
Findings, Conclusions, and Recommendations 1 2 5
Tires differ in their rolling resistance.
All tires cause rolling resistance, but to differing degrees. To improve traction and prolong wear, the tread component of the tire must have a substantial portion of the deformable, hysteretic material in the tire. The type and amount of material in the tread are therefore important determinants of rolling resistance. Other tire features and design parameters affect rolling resistance as well, including tire mass, geometry, and construction type.
About 80 percent, or 200 million, of the 250 million passenger tires shipped each year in the United States go to the replacement market, while the remaining 50 million are installed on new passenger vehicles as original equipment. There is considerable evidence to suggest that OE tires cause less rolling resistance, on average, than do replacement tires. Automobile manufacturers specify the tires installed on each of their vehicles; they tailor tire properties and designs to each vehicle’s appearance, suspension, steering, and braking systems. Rolling resistance is usually one of the specified properties since it can affect a vehicle’s ability to meet federal standards for fuel economy. Replacement tires, in contrast, are typically designed by tire manufacturers in a more general fashion to suit a wide range of in-use vehicles and a more diverse set of user requirements. The emphasis placed on characteristics such as traction, wear resistance, and rolling resistance can vary widely from tire to tire, depending on the demands of the specific segment of the replacement market.
Individual tires that start out with different rolling resistance—whether OE or replacement tires—will not retain the same differential over their service lives. Rolling resistance generally diminishes with tire use, and differences among tires will change. The many physical changes that tires undergo as they are used and age will modify rolling resistance over their life span. In particular, the loss of hysteretic tread material due to wear causes rolling resistance to decline. The rolling resistance of a properly inflatedtirewilltypicallydeclinebymorethan 20 percentoveritsservicelife.
Tire condition and maintenance have important effects on rolling resistance.
How well tires are maintained has a critical effect on their rolling resistance. Proper tire inflation is especially important in controlling rolling
1 2 6 Tires and Passenger Vehicle Fuel Economy
resistance because tires deform more when they are low on air. For typical passenger tires inflated to pressures of 24 to 36 pounds per square inch (psi), each 1-psi drop in inflation pressure will increase rolling resistance by about 1.4 percent. Hence, a drop in pressure from 32 to 24 psi—a significant degree of underinflation that would not be apparent by casually viewing the shape of the tire—increases a tire’s rolling resistance by more than 10 percent. At pressures below 24 psi, rolling resistance increases even more rapidly with declining inflation pressure. Tire misalignment and misbalancing are among other installation and maintenance factors that increase vehicle energy consumption from rolling resistance as well as other drag forces.
rolling resistance characteristics can be measured and compared.
By holding inflation pressure and other operating conditions constant, a tire’s rolling resistance characteristic can be measured for the purposes of design specification and comparisons with other tires. A tire’s rolling resistance characteristic is normally expressed as a rate, or coefficient, with respect to the wheel load (that is, the weight on each wheel). A tire’s rolling resistance increases in proportion to the wheel load.
The large majority of new passenger tires, properly inflated, have rolling resistance coefficients ranging from 0.007 to 0.014, with most having values closer to the average of about 0.01. Thus, the rolling resistance experienced by a passenger vehicle weighing 4,000 pounds with new tires may range from 28 to 56 pounds, or 7 to 14 pounds per tire. All else remaining constant, a vehicle equipped with a set of passenger tires having an average rolling resistance coefficient of 0.01 will consume about 1 to 2 percent less fuel than will a vehicle with tires having a coefficient of 0.011. Whether such a differential in fuel economy would be observed at all points in the lifetime of the two sets of tires will depend in large part on how their respective rolling resistance characteristics change with tire condition and tread wear.
has been made in reducing tire rolling resistance.
Significant progress has been made in reducing passenger tire rolling resistance during the past three decades through changes in tire designs, construction, and materials. The mass introduction of radial tires in the 1970s
Findings, Conclusions, and Recommendations 1 2 7
caused rolling resistance in new passenger tires to decline by about 25 percent. Subsequent changes in tire designs and materials have led to further reductions. Comparisons of the rolling resistance values of samples of new replacement radial tires sold today with those of radial tires sold 25 years ago show this progress. The lowest rolling resistance values measured in today’s new tires are 20 to 30 percent lower than the lowest values measured among replacement tires sampled during the early 1980s.
However, the spread in rolling resistance values has increased over time, which is attributable to a proliferation in tire sizes, types, and speed capabilities. The average rolling resistance measured for new tires has therefore not changed as dramatically: it has declined by about 10 percent during the past decade. For reasons related to their design and construction requirements, tires with high speed ratings tend to have higher-thanaverage rolling resistance. These tires have become more popular in the replacement market.
Rolling resistance is not governed by a single set of tire design and construction variables. Even when tires are grouped by common size and speed ratings, the difference in rolling resistance values among tires often exceeds 20 percent. The data suggest that many design and construction variables can be adjusted to influence rolling resistance.
Tires with lower rolling resistance and generally accepted traction capability are now on the market.
Tire rolling resistance and traction characteristics are related because they are both heavily influenced by the tire’s tread. The main function of the tread is traction, with thicker and deeper-grooved treads generally having better traction on wet, snowy, or otherwise contaminated road surfaces. Although a large amount of hysteretic material in the tread is usually advantageous for such traction capability, it can be a primary source of rolling resistance.
Passenger tires are rated for wet traction capability as part of the federal government’s Uniform Tire Quality Grading (UTQG) system. Data available to the committee on replacement tires indicate that tires with the highest UTQG traction grade (AA) typically have high speed ratings and are often marketed as very-high-performance tires. Such tires seldom exhibit lower-than-average rolling resistance. This relationship should be
1 2 8 Tires and Passenger Vehicle Fuel Economy
expected, since wet traction and responsive stopping capability are fundamental to the design and construction of very-high-performance tires. The large majority of tires in the marketplace, however, are designed to achieve the more modest UTQG system grade of A for traction. Among these tires, there is a much wider spread in rolling resistance values, and many such tires exhibit lower-than-average rolling resistance. Differences of 10 percent or more in rolling resistance are common among these tires, which suggests the technical feasibility and practicality of lowering rolling resistance while maintaining generally accepted levels of traction capability.
The relationship between tire rolling resistance and wear resistance depends on many tire design variables.
Tread wear is the main determinant of tire life. Shorter tire wear life results in more scrap tires and in consumers spending more on tire replacement, both of which are undesirable. Consequently, tire companies and their material suppliers have invested in research and development to find ways to reduce rolling resistance with minimal adverse effects on tread wear. The relationship between rolling resistance and wear resistance has been found to be determined by a combination of factors, including the type and amount of materials in the tread and the tread’s design and dimensions.
Numerous changes in tread materials and formulations, including modifications of polymers and carbon black fillers and the substitution of silica-silane fillers, have been examined with the intent of reducing rolling resistance with few adverse side effects. Because many of these systems are proprietary, their cost, levels of use, and effect on tread wear are not well documented. However, it is clear from observing OE tires, and their acceptance by automobile manufacturers, that much progress has been made over the past two decades in the development of technologies and systems to reduce rolling resistance. Further advances in OE tires are anticipated and are likely to flow into the replacement market.
Another apparent way to reduce rolling resistance is to build tires with less tread material. This could have adverse effects on wear life and traction. In practice, tire designers can reduce tread mass and volume through combinations of changes in tread depth, width, shoulder profile, and sec
Findings, Conclusions, and Recommendations 1 2 9
tion width. Data comparing rolling resistance and the single dimension of tread depth (the tread dimension that is most commonly listed for passenger tires) were examined in this study. They show that rolling resistance coefficients measured for new tires decline as tread depth declines. The data suggest that reducing new-tire tread depth by 2/32 inch, or almost 20 percent for the average tire in the study data set, will reduce new-tire rolling resistance coefficients by close to 10 percent. However, each reduction in tread depth of 1/32 inch is associated with lower UTQG tread wear ratings—about 5 percent lower on average. As might be expected, thinner tread is associated with shorter wear life, if compensating effects that may be achieved by altering materials and other tire design and construction technologies are disregarded.
Compared with an otherwise equivalent tire starting out with thicker tread, a tire starting out with thinner tread will yield fuel savings for a limited period. These savings will occur only during those miles traveled while the thicker-treaded tire is wearing down to the initial depth of the thinner-treaded tire. When the added tread thickness is gone, the two tires will essentially assume the same wear and rolling resistance profile per mile. The thinner-treaded tire will wear out sooner. Over its life, the tire starting out with less tread will exhibit slightly lower average rolling resistance per mile, but it will require earlier replacement at a cost to the motorist and lead to an increase in scrap tires.
Reducing rolling resistance saves fuel.
If the average rolling resistance exhibited by replacement tires in the passenger vehicle fleet were to be reduced by 10 percent, motorists would save $12 to $24 per year in fuel expenses, or roughly $1.20 to $2.40 for every 1 percent reduction in average rolling resistance. This assumes a long-term average price of $2 per gallon for gasoline and diesel fuel, as recently projected by the U.S. Department of Energy. The time required to achieve a 10 percent reduction in the average rolling resistance of replacement tires is not considered here but would depend on how the reduction is brought about. Presumably, it would require at least as many years as needed to turn over most passenger tires in the fleet, and perhaps added time for the development and introduction of any required technologies.
1 3 0 Tires and Passenger Vehicle Fuel Economy
Extrapolation to the 175 million passenger vehicles using replacement tires results in an estimate of national fuel savings ranging from $2 billion to $4 billion per year.
rolling resistance will have modest effects on tire expenditures.
The effect of reducing rolling resistance on consumer tire expenditures is difficult to estimate without knowing the precise magnitude of the reduction or how it would occur. A 10 percent reduction in the average rolling resistance of replacement tires on the road could occur through a combination of changes in the distribution of tires purchased and greater use being made of various technologies to reduce rolling resistance. It could also be achieved in part through more vigilant tire maintenance. Different approaches to achieving a reduction must be considered when effects on tire expenditures are estimated.
Data on new replacement tires do not show any clear pattern of price differences among tires that vary in rolling resistance but that are comparable in many other respects such as traction, size, and speed rating. This result suggests that consumers buying existing tires with lower rolling resistance will not necessarily pay more for these tires or incur higher tire expenditures overall, as long as average tire wear life is not shortened. Calculations in this report suggest that each 1 percent reduction in tire wear life costs consumers about $1.20 more per year in added tire expenses because of more frequent tire replacement. Consequently, a shift in the kinds of tires purchased that has the effect of reducing average rolling resistance but also reducing the average life of replacement tires will cause higher tire expenditures, as well as larger numbers of scrap tires. A reduction in average tire life of as little as 5 percent could cause an increase in tire expenditures that offsets all or a large portion of the savings in fuel. Because of such poor economics, reductions in tread depth and other measures to reduce rolling resistance that have significant impacts on tire wear life could be unwise and may be unacceptable.
Tire manufacturers and their suppliers have been actively researching new materials and technologies to reduce rolling resistance that will affect wear resistance and traction only minimally. These materials and technologies, many focused on tread composition, tend to be more costly to apply. However, rough estimates suggest a small addition to tire produc
Findings, Conclusions, and Recommendations 1 3 1
tion costs, on the order of $1 per tire. In practice, tread modifications designed to reduce rolling resistance tend to be applied as part of a broader array of changes in tire design, construction, and dimensions. The committee could not find detailed quantitative information on how such practical changes, in their many potential combinations, are likely to affect other aspects of tire performance such as traction and wear resistance.
Motorists currently purchase 200 million replacement tires per year. An increase in tire prices averaging $1 per tire would cost vehicle owners $200 million per year, if tire wear and replacement rates are held constant. Total national spending on replacement tires would thus increase in this instance by about $200 million per year. U.S. consumers have demonstrated a desire to maintain, and indeed extend, tire wear life, which suggests that poor wear performance would be unacceptable. If tire wear life were diminished on average, additional tire expenditures could greatly exceed $200 million per year, owing to the need for more frequent tire replacement.
If reductions in rolling resistance are achieved through more vigilant tire and inflation maintenance, tire wear life would be prolonged, and expenditures on tires by consumers would be reduced.
CONCLUSIONS IN RESPONSE TO STUDY CHARGE
Congress called for this study of the feasibility and effects of lowering the rolling resistance of replacement tires installed on cars and light trucks used for passenger transportation. Although many gaps in information and understanding persist, the findings and estimates presented above are helpful in answering the series of questions asked. Specifically, Congress asked how lowering replacement tire rolling resistance would affect
• Motor fuel use;
• Tire wear life and the creation of scrap tires;
• Tire performance characteristics, including those relevant to vehicle
safety; and
• Tire expenditures by consumers.
Drawing on the study findings, the committee offers its assessment of the feasibility of reducing rolling resistance and its conclusions in
1 3 2 Tires and Passenger Vehicle Fuel Economy
response to the individual elements of the study charge. The findings and conclusions, coupled with other insights gained during the course of the study, convince the committee that tire energy performance deserves greater attention from government, industry, and consumers. A recommendation for congressional action is offered in light of the following conclusions.
Feasibility of Lowering Rolling Resistance in Replacement Tires
Reducing the average rolling resistance of replacement tires by a magnitude of 10 percent is technically and economically feasible. A tire’s overall contribution to vehicle fuel consumption is determined by its rolling resistance averaged over its lifetime of use. A reduction in the average rolling resistance of replacement tires in the fleet can occur through various means. Consumers could purchase more tires that are now available with lower rolling resistance, tire designs could be modified, and new tire technologies that offer reduced rolling resistance could be introduced. More vigilant maintenance of tire inflation pressure will further this outcome. In the committee’s view, there is much evidence to suggest that reducing the average rolling resistance of replacement tires by a magnitude of 10 percent is feasible and attainable within a decade through combinations of these means.
Rolling resistance varies widely among replacement tires already on the market, even among tires that are comparable in price, size, traction, speed capability, and wear resistance. Consumers, if sufficiently informed and interested, could bring about a reduction in average rolling resistance by adjusting their tire purchases and by taking proper care of their tires once in service, especially by maintaining recommended inflation pressure. The committee does not underestimate the challenge of changing consumer preferences and behavior. This could be a difficult undertaking, and it must begin with information concerning the tire’s influence on fuel economy being made widely and readily available to tire buyers and sellers. A significant and sustained reduction in rolling resistance is difficult to imagine under any circumstances without informed and interested consumers.
The committee observes that consumers now have little, if any, practical way of assessing how tire choices can affect vehicle economy.
Findings, Conclusions, and Recommendations 1 3 3
Influence on Vehicle Fuel Economy
Tires and their rolling resistance characteristics can have a meaningful effect on vehicle fuel economy and consumption. A 10 percent reduction in average rolling resistance, if achieved for the population of vehicles using replacement tires, promises a 1 to 2 percent increase in the fuel economy of these vehicles. About 80 percent of passenger cars and light trucks are equipped with replacement tires. Assuming that the number of miles traveled does not change, a 1 to 2 percent increase in the fuel economy of these vehicles would save about 1 billion to 2 billion gallons of fuel per year of the 130 billion gallons consumed by the entire passenger vehicle fleet. This fuel savings is equivalent to the fuel saved by taking 2 million to 4 million cars and light trucks off the road. In this context, a 1 to 2 percent reduction in the fuel consumed by passenger vehicles using replacement tires would be a meaningful accomplishment.
Effects on Tire Wear Life and Scrap Tires
The effects of reductions in rolling resistance on tire wear life and scrap tires are difficult to estimate because of the various ways by which rolling resistance can be reduced. The tread is the main factor in tire wear life and the main component of the tire contributing to rolling resistance. Reductions in tread thickness, volume, and mass are among the means available to reduce rolling resistance, but they may be undesirable if they lead to shorter tire lives and larger numbers of scrap tires. Various tread-based technologies are being developed and used with the goal of reducing rolling resistance without significant effects on wear resistance. The practical effects of these technologies on tread wear and other tire performance characteristics have not been established quantitatively. However, continuing advances in tire technology hold much promise that rolling resistance can be reduced further without adverse effects on tire wear life and scrap tire populations.
Effects on Traction and Safety Performance
Although traction may be affected by modifying a tire’s tread to reduce rolling resistance, the committee could not find safety consequences. Such consequences may be undetectable. Changes are routinely made
1 3 4 Tires and Passenger Vehicle Fuel Economy
in tire designs, materials, and construction methods for reasons ranging from noise mitigation and ride comfort to steering response and styling. All can have implications for other tire properties and operating performance, including traction capability. Discerning the safety implications of small changes in tire traction characteristics associated with tread modifications to reduce rolling resistance may not be practical or even possible, especially since there is no single way to reduce rolling resistance. The committee could not find safety studies or vehicle crash data that provide insight into the safety impacts associated with large changes in traction capability, much less the smaller changes that may occur from modifying the tread to reduce rolling resistance.
Effects on Consumer Fuel and Tire Expenditures
Reducing the average rolling resistance of replacement tires promises fuel savings to consumers that exceed associated tire purchase costs, as long as tire wear life is not shortened. A 10 percent reduction in rolling resistance can reduce consumer fuel expenditures by 1 to 2 percent for typical vehicles. This savings is equivalent to 6 to 12 gallons per year, or $12 to $24 if fuel is priced at $2 per gallon. Tire technologies available today to reduce rolling resistance would cause consumers to spend slightly more when they buy replacement tires, on the order of $1 to $2 per year. These technologies, however, may need to be accompanied by other changes in tire materials and designs to maintain the levels of wear resistance that consumers demand. While the effect of such accompanying changes on tire production costs and prices is unclear, the overall magnitude of the fuel savings suggests that consumers would likely incur net savings in their expenditures.
RECOMMENDATIONS TO INFORM CONSUMERS
As a general principle, consumers benefit from the ready availability of easy-to-understand information on all major attributes of their purchases. Tires are no exception, and their influence on vehicle fuel economy is an attribute that is likely to be of interest to many tire buyers. Because tires are driven tens of thousands of miles, their influence on vehicle fuel consumption can extend over several years. Ideally, consumers
Findings, Conclusions, and Recommendations 1 3 5
would have access to information that reflects a tire’s effect on fuel economy averaged over its anticipated lifetime of use, as opposed to a measurement taken during a single point in the tire’s lifetime, usually when it is new. No standard measure of lifetime energy consumption is currently available, and the development of one deserves consideration. Until such a practical measure is developed, rolling resistance measurements of new tires can be informative to consumers, especially if they are accompanied by reliable information on other tire characteristics such as wear resistance and traction.
Advice on specific procedures for measuring and rating the influence of individual passenger tires on fuel economy and methods of conveying this information to consumers is outside the scope of this study. Nevertheless, the committee is persuaded that there is a public interest in consumers having access to such information. The public interest is comparable with that of consumers having information on tire traction and tread wear characteristics, which is now provided by industry as required by the federal Uniform Tire Quality Grading standards.
It is apparent that industry cooperation is essential in gathering and conveyingtireperformanceinformationthatconsumerscanuseinmaking tire purchases. It is in the spirit of prompting and ensuring more widespread industry cooperation in the supply of useful and trusted purchase information that the committee makes the following recommendations.
Congress should authorize and make sufficient resources available to NHTSA to allow it to gather and report information on the influence of individual passenger tires on vehicle fuel consumption. Information that best indicates a tire’s contribution to vehicle fuel consumption and that can be effectively gathered, reported, and communicated to consumers buying tires should be sought. The effort should cover a large portion of the passenger tires sold in the United States and be comprehensive with regard to popular tire sizes, models, and types, both imported and domestic.
NHTSA should consult with the U.S. Environmental Protection Agency on means of conveying the information and ensure that the information is made widely available in a timely manner and is easily understood by both buyers and sellers. In the gathering and
1 3 6 Tires and Passenger Vehicle Fuel Economy
communication of this information, the agency should seek the active participation of the entire tire industry.
The effectiveness of this consumer information and the methods used for communicating it should be reviewed regularly. The information and communication methods should be revised as necessary to improve effectiveness. Congress should require periodic assessments of the initiative’s utility to consumers, the level of cooperation by industry, and the resultant contribution to national goals pertaining to energy consumption.
Finally, even as motorists are advised of the energy performance of tires, they must appreciate that all tires require proper inflation and maintenance to achieve their intended levels of energy, safety, wear, and operating performance. As new technologies such as tire pressure monitoring systems, more energy-efficient tire designs, and run-flat constructions are introduced on a wider basis, they must have the effect of prompting more vigilant tire maintenance rather than fostering more complacency in this regard. Motorists must be alerted to the fact that even small losses in inflation pressure can greatly reduce tire life, fuel economy, safety, and operating performance. A strong message urging vigilant maintenance of inflation must therefore be a central part of communicating information on the energy performance of tires to motorists.
A P P E N D I X
Explanation and Comparison of Society of Automotive Engineers Test Procedures for Rolling Resistance
MARION G. POTTINGER
M’gineering, LLC
Two standardized tests are used in the United States to measure the rolling resistance of tires. The two tests are detailed in recommended practices of the Society of Automotive Engineers (SAE): J1269, “Rolling Resistance Measurement Procedure for Passenger Car, Light Truck, and Highway Truck and Bus Tires,”1 and J2452, “Stepwise Coastdown Methodology for Measuring Tire Rolling Resistance.” J1269 is the older of the two practices. It was approved in 1979 and reaffirmed in 2000. J1269 is intended to “provide a way of gathering data on a uniform basis, to be used for various purposes (for example, tire comparisons, determination of load and pressure effects, correlation with test results from fuel consumption tests, etc.).”2 J2452 was approved by SAE in 1999. Its primary
intent is “estimation of the tire rolling resistance contribution to vehicle force applicable to SAE Vehicle Coastdown recommended practices J2263 and J2264.”3
COMMON FEATURES OF THE TWO TEST PRACTICES
The two practices have common features such as test wheel diameter, surface texture, and ambient temperature. The commonalities are noted in
1
J1269 is accompanied by an information report, J1270, “Measurement of Passenger Car, Light Truck, and Highway Truck and Bus Tire Rolling Resistance.”
2
The quotation is drawn from the J1269 document.
3
The quotation is drawn from the J2452 document.
1 3 7
1 3 8 Tires and Passenger Vehicle Fuel Economy
TABLE A-1 Items Common to J1269 and J2452
Item Specification
Test wheel diameter 1.7 m (67 in.)
Measurement methodsa Force
Torque
Surface 80-grit paperb
Allowed ambient temperature 20°C (68°F) ≤ T ≤ 28°C (82°F)
Reference temperature 24°C (75°F)
a J1269 also allows rolling resistance determination by measure
ment of electrical power consumption, but this method is no longer in common use.
b This is actually an emery cloth. J2452 contains a surface con
ditioning procedure for the material.
Table A-1. The practices use the same test rims. The normally used test rims are the measuring rims,4 but other rims approved in a tire and rim
standards organization yearbook such as that of the Tire and Rim Association may be used. The rim used is always noted in the test report because rim width affects test results.
DIFFERENCES BETWEEN THE TWO PRACTICES
There are a number of differences between the two practices, which are detailed below.
Inflation Pressure and Load
Tire rolling resistance is dependent on inflation pressure and load. In both test practices inflation pressure is defined in terms of a base pressure. Base pressure is not defined in precisely the same manner in the two practices. In J1269 it is the inflation pressure molded on the tire sidewall together with the maximum load. This is straightforward for P-tires, but it only applies to single-tire loading in the case of LT-tires.5 In J2452, P-tire
base pressures are defined in the first table in the recommended practice.
4 The design/measuring rim is the specific rim assigned to each specific tire designation to deter
mine basic tire dimensions. This rim is specified for each tire designation in the yearbooks of tire and rim standards organizations such as the Tire and Rim Association, Inc.
5 P-tires are passenger tires. LT-tires are light truck tires.
Explanation and Comparison of Test Procedures for Rolling Resistance 1 3 9
They are different from those given in J1269 for some tires. The base pressure for LT-tires matches that given in J1269.
In both practices load is defined in terms of maximum load. “Maximum load” is defined in both practices as the maximum load molded on the tire sidewall and listed as the load limit in the tire load tables of the current yearbook for the relevant tire and rim standards organization. For LT-tires this is the maximum load for single-tire operation.
Test Elements
Test elements include break-in, warm-up, and the actual test conditions. Break-in is to be used with tires that change in dimensions or material properties during first operation. Break-in is usually not required since the first 30 minutes of warm-up for Test Condition 1 is considered to be an allowable substitute for formal break-in. Also, until the tire has passed through first operation, there is no way to determine whether it will change in dimensions or material properties. Furthermore, since the load and inflation for Test Condition 1 in J1269 and J2452 are not the same, the resultant effective break-in is Recommended Practice-specific.
During the warm-up process, which occurs before each test condition, the tire is brought to thermal equilibrium. There are two approved ways to perform the warm-up: timed and rolling resistance force rate of change determined. In the timed method the tire is operated for a defined time at the conditions for each test step before data acquisition for that step. For P-tires the time period before Condition 1 is 30 minutes. It is 10 minutes before other steps. For LT-tires the period before Condition 1 is 60 minutes. It is 15 minutes before other steps. In the rate of change method, after a short waiting period for Condition 1 (10 minutes for P-tires and 20 minutes for LT-tires) and without a waiting period for other conditions, the rolling resistance is monitored with equilibrium being defined to exist when the rolling resistance gradient is less than or equal to 0.13 newtons per minute over a 90-second period. Regardless of the warm-up method, once equilibrium formally exists for each condition, data acquisition can begin.
The test conditions used for P-tires are defined in Table A-2, and those for LT-tires are defined in Table A-3. The test conditions for J1269 and J2452 are not identical. The exact procedure for executing the test under the test conditions is discussed under the subject of test execution.
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TABLE A-2 Regulated Pressure Test Conditions for P-Tires
J1269 J2452
Test Point % Max Load Base Pressure ± (kPa) % Max Load Base Pressure ± (kPa)
1 90 −30 30 +10
2 90 +70 60 −40
3 50 −30 90 +60
4 50 +70 90 −40
NOTE: There is a version of the J1269 procedure in which Step 1 is conducted under capped condi
tions. In this case, the inflation pressure is established cold, the valve cap is put in place, and all increases in pressure are due to the rising tire temperature during the warm-up period.
HANDLING OF DATA CORRECTIONS
Raw data taken during testing contain tares (offsets), parasitic losses such as bearing losses, force measurement crosstalk, and perhaps alignment errors. Additional data besides the basic data acquired according to the section on test execution are required to eliminate these errors. These correction data are used during data analysis.
The load cell output with the test tire and rim mounted but not loaded is acquired for each test condition to obtain tares. During analysis, these data are subtracted from the data taken for the test condition to which they pertain.
With the tire loaded just enough so that it will continue to rotate, force or torque data, whichever are relevant for the test machine being used,
TABLE A-3 Regulated Pressure Test Conditions for LT-Tires
J1269 J2452
Test Point % Max Load % Base Pressure % Max Load % Base Pressure
1 100 110 20 110
2 70 60 40 50
3 70 110 40 100
4 40 30 70 60
5 40 60 100 100
6 40 110
Explanation and Comparison of Test Procedures for Rolling Resistance 1 4 1
are acquired for each speed. These data contain the parasitic bearing losses and aerodynamic losses. During analysis, these data are subtracted from the data taken for the test condition to which they pertain.
Crosstalk occurs in all multidimensional force measurement machines. A matrix to remove this effect is derived during machine calibration. If errors exist because of machine load application alignment imperfections not fully compensated by the crosstalk matrix, the test must be run in both directions of rotation on force measurement rolling resistance test machines, and the results must be averaged.
HOW THE TESTS ARE EXECUTED J1269
With the test machine operating at a steady 80 km/h, data are acquired
according to the following sequence:
• Warm-up at P1 and FZ 1.
• Acquire data at P1 and FZ 1.
• Warm-up at P2 and FZ 2. • Acquire data at P2 and FZ 2.
• Acquire data at Pn and FZn as prescribed in the relevant practice. J2452
For each test condition, the tire is warmed up at 80 km/h until steadystate rolling resistance is achieved. At that point the tire is quickly accelerated to 115 km/h and then subjected to a stepwise approximation to a 180-second coastdown to 15 km/h. The stepwise approximation contains
six or more approximately equally spaced steps. Figure A-1 is an example of such a coastdown.
COMMON DATA ANALYSIS
The first step is to apply the required data corrections. At that point the
rolling resistance is computed. Next the data are adjusted to give the rolling resistance at 24°C (75°F) by using Equation 1.
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FIGURE A-1 Example of stepwise coastdown in J2452 test practice.
where
RRT = rolling resistance at 24°C,
RR = rolling resistance at TA,
TA = ambient temperature during a test condition, and
TR = reference temperature = 24°C.
The k-values given in J1269 and J2452 are not the same. Since the data are taken on a 1.7-meter-diameter test dynamometer, they are not correct for other diameters, for example, ∞ (flat) or 1.22 meters (48 inches), which is used in federal vehicle emission and fuel economy tests. An approximate correction for curvature is obtained by applying the Clark equation, Equation 2.6 Equation 3 is the Clark equation for the
special case of a flat surface.
6
The text of J2452 notes that the question of correction for curvature needs to be revisited; however, this has not been done since J2452 was adopted in 1999.
Explanation and Comparison of Test Procedures for Rolling Resistance 1 4 3
where
R1 = measurement surface radius,
R2 = radius of the surface to which the data are being adjusted, and
r = unloaded tire nominal radius.
DATA FITTING
For modeling and other engineering purposes, empirical relationships are fit by using the J1269 and J2452 data. Because consistency with J1269 was not considered during the development of J2452, the J2452 equation does not devolve to the J1269 equation when velocity is set to 80 km/h. J1269 was not revised so that its equations are the J2452 equation at a single velocity.
For J1269 P-tire fitting,
For J1269 LT-tire fitting,
In Equations 4 and 5, FZ is load, P is inflation pressure, and A0, A1, . . . , A4 are constants.
For J2452 fitting,
where
a, b, c, α, β = constants;
FZ = load;
P = inflation pressure; and V = speed.
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SINGLE-NUMBER EXPRESSION OF RESULTS
In comparing tire specifications, it is important to be able to characterize tire rolling resistance with a single number. The model derived from J1269 or J2452 can be queried to yield a rolling resistance value at a single point.
Simplified Standard Reference Condition
Because of the possibility of needing to produce data on a large array of tires, J2452 contains a Simplified Standard Reference Condition, which yields data at the following single condition.
• Load = 70 percent of maximum, • Inflation = base + 20 kPa, and • V = 80 km/h.
(At the time this appendix was prepared, a single-point test at the Simplified Standard Reference Condition was in ballot as a revision of J1269.)
Mean Equivalent Rolling Force
J2452 contains a method for deriving a single number representative of a known driving cycle. This is the mean equivalent rolling force (MERF). It is calculated by Equation 7.
where
RR = rolling resistance as a function of time within the chosen cycle,
tf = final time in the cycle, and
t0 = initial time in the cycle.
Equation 7 is the time integration of the rolling resistance during the cycle under study divided by the time during which the cycle occurs.
Explanation and Comparison of Test Procedures for Rolling Resistance 1 4 5
Typically, the cycle under consideration would be one of the federal test procedure (FTP) driving cycles such as the urban or highway schedule. If MERF is computed for both FTP cycles, a MERF related to corporate average fuel economy (CAFE) can be computed as indicated in Equation 8.
MERF CAFE = . 0 55 ( MERF URBAN ) + . 0 45 ( MERF HIGHWAY ) (8)
Standard MERF
This is a MERF computed at the standard reference conditions discussed above.
Study Committee
Biographical Information
Dale F. Stein, Chair, is President Emeritus of Michigan Technological University. He has also been Vice President of Academic Affairs and Professor in the Departments of Metallurgical Engineering and Mining Engineering. He began his career as a Research Metallurgist at General Electric Research Laboratory. His major research interests are in the deformation and fracture of materials and the relationship between materials and the environment. Dr. Stein has an interest in the recycling and efficient use of materials. He was a pioneer in the application of auger spectroscopy to the solution of metallurgical problems and a leading authority on the mechanical properties of engineering materials. He is a Fellow of the Minerals, Metals and Materials Society; the American Society for Metals; and the American Association for the Advancement of Science. He has been a member of more than 20 committees and panels of the National Academies. He has chaired many of these committees, including the Committee on Novel Approaches to the Management of Greenhouse Gas Emissions from Energy Systems, the Committee on Materials Science and Engineering, and the Transportation Research Board’s (TRB’s) Research and Technology Coordinating Committee for the Federal Highway Administration. He was a member of the National Materials Advisory Board. Dr. Stein was elected to the National Academy of Engineering in 1986. He holds a BS in metallurgy from the University of Minnesota and a PhD in metallurgy from Rensselaer Polytechnic Institute.
James E. Bernard is Anson Marston Distinguished Professor of Engineering at Iowa State University and Director of the Virtual Reality Applications Center. His research interests include vehicle dynamics and driving simulation, and he is a member of the Vehicle Dynamics Subcommittee of the Society of Automotive Engineers (SAE). He has written numerous
Study Committee Biographical Information 1 4 7
papers relating to motor vehicle rollover and associated vehicle test methods, including a comprehensive literature review. Dr. Bernard has received a number of awards for his contributions to graduate and undergraduate teaching, including the SAE Ralph R. Teetor Award for “significant contributions to teaching, research and student development.” He has received awards for his technical research papers from Tire Science and Technology and the MSC.Nastran World Users Conference. He was a member of the National Academies Committee for the Motor Vehicle Rollover Rating System Study. He held teaching and research positions at the University of Michigan and Michigan State University before joining the faculty of Iowa State University as Professor and Chairman of Mechanical Engineering in 1983. He received his BS, MS, and PhD in engineering mechanics from the University of Michigan.
John Eagleburger retired in 2003 as Manager of Products Adjustments and Claims Performance for Goodyear Tire Company. From 1995 to 2002, he was Leader of the General Motors (GM) Team of the Akron Technical Center, where he managed a multidisciplinary engineering team in the design, testing, and approval of original equipment manufacturer passenger and light truck tires for GM vehicles. From 1988 to 1995, he was based in Tokyo as Goodyear’s Manager of Engineering and supplied tire products to Japanese and Korean automobile makers. He was previously GM account manager for Goodyear based in Detroit and manager of technical coordination for tire standards. Mr. Eagleburger began his career at Goodyear in 1965, serving as a tire design engineer and project engineer. He was active in SAE and served on several technical committees during his career. He holds a BS in mechanical engineering from the University of Wisconsin.
Richard J. Farris is Distinguished University Professor Emeritus at the Silvio Conte National Center for Polymer Research in the Polymer Science and Engineering Department, University of Massachusetts, Amherst. His research interests are in experimental mechanics, highperformance fibers, rubber elasticity and thermodynamics, particulate composites, and recycling of elastomers. He has more than 300 refereed publications and 16 patents. Dr. Farris served as Chairman of the Gordon Research Conference on Composites and the Gordon Research
1 4 8 Tires and Passenger Vehicle Fuel Economy
Conference on High-Performance Thermo-Setting Materials and as a member of numerous advisory committees for the National Aeronautics and Space Administration and other government agencies. He is a Fellow of the Society of Plastics Engineers and served as a member of the National Academies Panel on Structural and Multifunctional Materials. He is the recipient of the Roon Award of the Federation of Societies for Coating Technology (1998), the Malcolm Pruitt Award of the Council for Chemical Research (2003), the George Stafford Whitby Award from the Rubber Division of the American Chemical Society (2005), and the Founder’s Award from the Society of Plastics Engineers (2006). He holds an MS and a PhD in civil engineering from the University of Utah.
David Friedman is Research Director for the Union of Concerned Scientists’ (UCS) Clean Vehicles Program. He is the author or coauthor of more than 30 technical papers and reports on advances in conventional, fuel cell, and hybrid electric vehicles and alternative energy sources with an emphasis on clean and efficient technologies. Before joining UCS in 2001, he worked for the University of California at Davis (UC Davis) in the Fuel Cell Vehicle Modeling Program, where he developed simulation tools to evaluate fuel cell technology for automotive applications. He worked on the UC Davis FutureCar team to build a hybrid electric family car that doubled fuel economy. He previously worked at Arthur D. Little researching fuel cell, battery electric, and hybrid electric vehicle technologies, as well as photovoltaics. Mr. Friedman is a member of the Board on Energy and Environmental Systems’ Panel on Prospective Benefits of the Department of Energy’s Light-Duty Hybrid Vehicle R&D Program and previously served on that board’s Panel on the Prospective Benefits of the Department of Energy’s Fuel Cell R&D Program. He earned a bachelor’s degree in mechanical engineering from Worcester Polytechnic Institute and is a doctoral candidate in transportation technology and policy at UC Davis.
Patricia S. Hu is Director of the Center for Transportation Analysis at the Engineering Science and Technology Division of Oak Ridge National Laboratory (ORNL). She has been at ORNL since 1982 and in her current position since 2000. At ORNL, she has led many projects in transportation statistics and analysis. She chairs TRB’s Standing Committee on National Data Requirements and Programs and serves on other TRB standing com
Study Committee Biographical Information 1 4 9
mittees. She served on the Editorial Advisory Board of the international journal Accident Analysis and Prevention from 1996 to 1998 and has served on the Editorial Advisory Board of the Journal of Transportation Statistics since 1998. She led a team supported by the Transportation Security Administration studying the domain awareness of U.S. food supply chains by linking and analyzing geospatial data on transportation networks, traffic volume, choke points, freight flow, and traffic routing. She holds a bachelors degree from the National Chengchi University, Taipei, Taiwan, and an MS in mathematics and statistics from the University of Guelph, Ontario, Canada.
Wolfgang G. Knauss is Theodore von Kármán Professor of Aeronautics and Applied Mechanics at the California Institute of Technology. His work has centered on understanding the mechanics of time-dependent fracture in polymeric materials. He has served on several national committees and delegations, including the National Committee on Theoretical and Applied Mechanics (as chair), the U.S. delegation to the International Union of Theoretical and Applied Mechanics General Assembly, the Army Panel on Air and Ground Vehicle Technology, and the Aerospace Scientific Advisory Committee. Dr. Knauss has received numerous awards during his academic career, including Woodrow Wilson Foundation Fellowship and National Aeronautics and Space Administration Fellowship. He is an Elected Fellow of the Institute for the Advancement of Engineering, the American Society of Mechanical Engineers (ASME), and the National Academy of Mechanics. He was elected to the National Academy of Engineering in 1998 for engineering work on time-dependent fracture of polymers at interfaces and under dynamic loading. He was awarded the Senior U.S. Scientist Award by the Alexander von Humboldt Foundation and the Murray Medal of the Society of Experimental Mechanics. He holds a BS, an MS, and a PhD from the California Institute of Technology.
Christopher L. Magee is Professor of the Practice, Mechanical Engineering, at the Massachusetts Institute of Technology (MIT). He is also Engineering Systems Director at MIT’s multidisciplinary Center for Innovation in Product Development. Before joining MIT in 2002, he worked for the Ford Motor Company, beginning in the Scientific Research
1 5 0 Tires and Passenger Vehicle Fuel Economy
Laboratory and progressing through a series of management positions to Executive Director of Programs and Advanced Engineering. In the latter position, he had responsibility for all major technically advanced areas in Ford’s product development organization. During his career at Ford, he made major contributions to the understanding of the transformation, structure, and strength of ferrous materials. He developed lightweight materials for automobile manufacturing and pioneered experimental work on high-rate structural collapse to improve vehicle crashworthiness. He initiated Ford’s computer-aided engineering for structural and occupant simulation for crashworthiness. Dr. Magee is internationally recognized for this work and received the Alfred Nobel Award of the American Society of Civil Engineers. He was elected to the National Academy of Engineering in 1996. He has served on several National Academies committees, including the Panel on Materials Research Opportunities and Needs in Materials Science and Engineering and the Panel on Theoretical Foundations for Decision Making in Engineering Design. He currently serves on the Committee on Review of the FreedomCAR and Fuel Research Program. He earned a BS and a PhD from Carnegie-Mellon University and an MBA from Michigan State University.
Marion G. Pottinger retired in 2003 as Technical Director, Smithers Scientific Services, Inc. He is now a private consultant. Smithers Scientific Services, where he worked for 15 years, is an independent testing, research, and consulting firm. Before joining Smithers, he was Associate Research and Development Fellow at Uniroyal Goodrich Tire Company, where he focused on the development of high-performance tires and instruments to measure tire wear. Before 1985, he was a senior manager in the BFGoodrich research and development unit, responsible for research in acoustics, vibration, vehicle dynamics, tire force and moment, wear, and structural mechanics. Dr. Pottinger has published more than 50 articles and book chapters on tires, gearing, high-performance composites, and instruments. He is President Emeritus of the Tire Society and a member of SAE’s Highway Tire Forum Committee, Vehicle Dynamics Standards Committee, and Chassis and Suspension Committee. He is a member of the ASTM F-09 Committee on Tires. He holds a BS in mechanical engineering from the University of Cincinnati and an MS and a PhD in mechanical engineering from Purdue University.
Study Committee Biographical Information 1 5 1
Karl J. Springer is retired Vice President of Automotive Products and Emissions Research at Southwest Research Institute. He oversaw a staff of more than 600 employees engaged in research, testing, and evaluation of diesel and gasoline engine lubricants, fuels, fluids, emissions, and components for automotive, truck, bus, and tractor products. His research interests have focused on the measurement and control of air pollution emissions from on-road and off-road vehicles and equipment powered by internal combustion engines. Mr. Springer has authored more than three dozen peer-reviewed technical papers and publications. He was elected a member of the National Academy of Engineering in 1996 and is a Fellow of ASME, a Fellow of SAE, and a Diplomate of the American Academy of Environmental Engineers. He was named Honorary Member of ASME in 2003 for developing test methods for measuring emissions of smoke, odor, and particulate matter from internal combustion engines and advancing this understanding through extensive publishing activity. He is a recipient of ASME’s Honda Medal and Dedicated Service Award. He served on the National Academies Committee on Carbon Monoxide Episodes in Meteorological and Topographical Problem Areas and is a member of the Committee on State Practices in Setting Mobile Source Emissions Standards. He holds a BS in mechanical engineering from Texas A&M University and an MS in physics from Trinity University.
Margaret A. Walls is Resident Scholar at Resources for the Future (RFF). She was on the economics faculty of Victoria University, Wellington, New Zealand, from 1998 to 2000 and a Fellow in RFF’s Energy and Natural Resources Division from 1987 to 1996. Her current research focuses on solid waste and recycling, urban land use, and air quality issues. She has published numerous articles that assess the efficiency and effectiveness of solid waste policies. In the area of transportation, Dr. Walls has modeled household vehicle ownership and use and the cost-effectiveness of various alternative fuels. She has published more than two dozen articles in refereed journals and a dozen book chapters on energy, waste disposal, and land use policies. She is a member of the American Economics Association and the Association of Environmental and Resource Economists. She holds a BS in economics from the University of Kentucky and a PhD in economics from the University of California, Santa Barbara.
1 5 2 Tires and Passenger Vehicle Fuel Economy
Joseph D. Walter retired in 1999 as President and Managing Director of Bridgestone Technical Center Europe (Rome). He is now an Adjunct Professor in the College of Engineering at the University of Akron, where he teaches engineering mechanics courses. Before joining the Bridgestone Technical Center in 1994, he was Vice President and Director of Research at Bridgestone/Firestone, Inc. He began his career at the Firestone Tire and Rubber Company in 1966, where he held a series of technical and management positions of increasing responsibility. Dr. Walter has authored or coauthored more than two dozen journal articles and book chapters on aspects of tire mechanics, materials, design, and testing. He recently coedited the book The Pneumatic Tire, with a grant from the National Highway Traffic Safety Administration. He is a founding member of the Tire Society and is active in SAE, the American Chemical Society, and ASME. He was a member of the National Academies Committee on Fuel Economy of Automobiles and Light Trucks. He holds a BS, an MS, and a PhD in mechanical engineering from Virginia Polytechnic Institute and State University and an MBA from the University of Akron.
TIRES AND PASSENGER VEHICLE FUEL ECONOMY
Informing Consumers, Improving Performance
Every 3 to 5 years, the typical U.S. automobile owner chooses new tires that will affect the vehicle’s handling, traction, ride, and appearance, as well as its fuel economy. Annually $20 billion is spent on approximately 200 million replacement tires for personal vehicles, and an equal number of used tires are discarded. The collective outcome of these consumer choices about tires is a matter of public interest and national concern.
This report assesses the feasibility of reducing rolling resistance in replacement tires and examines the effects on vehicle fuel consumption, tire wear life, scrap tires, and operating performance safety.
Also of Interest
Consideration of Environmental Factors in Transportation Systems Planning
National Cooperative Highway Research Program (NCHRP) Report 541, ISBN 0-309-08839-9, 108 pages, 8.5 x 11, paperback, 2005, $24.00
Integrating Sustainability into the Transportation Planning Process
TRB Conference Proceedings 37, ISBN 0-309-09418-6, 59 pages, 8.5 x 11, paperback, $33.00
Car-Sharing: Where and How It Succeeds
Transit Cooperative Research Program (TCRP) Report 108, ISBN 0-309-08838-0, 246 pages, 8.5 x 11, paperback, 2005, $41.00
Review of the Research Program of the FreedomCAR and Fuel Partnership: First Report
National Research Council, National Academies Press, ISBN 0-309-09730-4, 146 pages, 8.5 x 11, paperback, 2005, $33.25
Predicting Air Quality Effects of Traffic-Flow Improvements: Final Report and User’s Guide
NCHRP Report 535, ISBN 0-309-08819-4, 227 pages, 8.5 x 11, paperback, 2005, $28.00
The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs
National Academy of Engineering, National Academies Press, ISBN 0-309-09163-2, 240 pages, 8 1/2 x 11, paperback, 2004, $32.00
Travel Matters: Mitigating Climate Change with Sustainable Surface Transportation
TCRP Report 93, ISBN 0-309-08773-2, 77 pages, 8.5 x 11, paperback, 2003, $20.00
ISBN 0-309-09421-6
Transportation
Monday, March 10, 2008
Tires directly impact fuel consumption
