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.
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.
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 inflated tire will typically decline by more than 20 percent over its service life.
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
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.
Tire 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.
Progress 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
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-than-average 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
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-
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.
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.
Reducing 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-
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
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.
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
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
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 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.
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.