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-
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
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.
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
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 high-performance 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-
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
Presentations and a summary of the IEA conference can be found at www.iea.org.
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
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.
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.
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.