The rapid rise in gasoline and diesel fuel prices experienced during 2005-2007 because of large increases in global demands and Middle East oil producers’ policies on oil production continues to make vehicle fuel economy an important policy issue, and growing recognition of the climate-change issue has drawn more attention to fuel economy. The recently passed Energy Independence and Security Act of 2007 requires DOT to raise vehicle fuel-economy standards, starting with model year 2011, until they achieve a combined average fuel economy of at least 35 mpg for model year 2020. A recent Supreme Court decision also requires EPA to regulate greenhouse-gas emissions from new light-duty vehicles under its Clean Air Act authority.1 DOT, through NHTSA, has continued to review estimates of the potential for various technologies to improve fuel economy. And a number of other investigations have been conducted to assess fuel economy or greenhouse-gas reduction potential, especially for California’s recent initiative to reduce greenhouse-gas emissions in the state.

NHTSA would like to keep up to date on the potential for technologic improvements as it moves into planned regulatory activities. It was as part of its technologic assessment that NHTSA asked the National Academies to update the 2001 National Research Council report and add to its assessment other technologies that have emerged since that report was prepared. The task statement directs the committee to estimate the efficacy, cost, and applicability of technologies that might be used over the next 15 years. The list of technologies includes diesel and hybrid electric powertrains, which were not considered in the 2001 assessment. Weight and power reductions also are to be included. Updating the fuel economy-cost curves for different vehicle size classes that are in Chapter 3 of the 2001 report is central to the request. The current study focuses on technology and will not consider CAFE issues related to safety, economic effects on industry, or the structure of fuel-economy standards; these issues were addressed in the earlier report. It will look at lowering fuel consumption by reducing power requirements through such measures as reduced vehicle weight, lower tire rolling resistance, or improved vehicle aerodynamics and accessories; by reducing the amount of fuel needed to produce the required power through improved engine and transmission technologies; by recovering some of the exhaust thermal energy with turbochargers and other technologies; and by improving engine performance and recovering energy through regenerative braking in hybrid vehicles.

This letter constitutes the interim report called for in the task statement. It discusses the technologies to be analyzed in the final report, the types of vehicles that may use them, the estimated improvements in fuel economy that may result, and the computational models that will be used in analysis. In producing this interim report, the committee met four times and received presentations from automobile manufacturers, suppliers of technologies to the automobile industry, federal agencies, and researchers in universities and government laboratories. The agendas for the committee’s public sessions are shown in Appendix D and demonstrate the committee’s commitment to hearing from diverse experts.

There are two primary ways to show the effectiveness with which fuel is used in vehicles: fuel economy and fuel consumption. Both are used in this report. Fuel-economy standards are expressed as miles driven per gallon of fuel consumed. Fuel consumption is the inverse measure: the amount of fuel consumed in driving a given distance, such as gallons consumed per 100 miles traveled. Fuel consumption is a fundamental engineering measure and is useful because it is related directly to the goal of decreasing the amount of fuel required to travel a given distance.2 Figure 1 shows how the two measures—miles per gallon and gallons per 100 miles—are related to one another: an increase in fuel economy from 25 mpg to 40 mpg is a 60% improvement in fuel economy and a 38% improvement in fuel consumption. Note that the curve is relatively flat beyond 35 mpg. Although this report describes changes primarily in terms of fuel consumption, in many cases it also expresses them in terms of fuel economy.


See Massachusetts et al. v. Environmental Protection Agency et al. EPA can avoid promulgating regulations only if it determines that greenhouse gases do not contribute to climate change or if it provides some reasonable explanation as to why it cannot or will not exercise its discretion to determine whether they do. Such an explanation must be grounded in the provisions of the Clean Air Act, not based on policy or other grounds.


Furthermore, the “average” in CAFE standards is determined as the sales-weighted average of fuel consumption converted into fuel economy.

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement