Direct from the US Federal Register: "The Environmental Protection Agency (EPA) is proposing changes to the test methods used to calculate the fuel economy estimates that are posted on window stickers of all new cars and light trucks sold in the United States. A fundamental issue with today's fuel economy estimates is that the underlying test procedures do not fully represent real-world driving conditions. Although no single test or set of tests can ever account for the wide variety of conditions experienced by every driver, the new fuel economy estimates would more accurately reflect a number of important factors that drivers are likely to experience on the road. These changes will take effect starting with 2008 model year vehicles. Under the new methods, the City MPG estimates for most vehicles would drop 10 percent to 20 percent from today's labels, depending on the vehicle. The Highway MPG estimates would generally drop 5 percent to 15 percent for most vehicles. Although today's proposed fuel economy test methods would provide more accurate estimates for many consumers, there will always continue to be drivers who get higher or lower fuel economy than the window sticker numbers. Currently the same test procedures are used for both the window sticker estimates and the fuel economy values used to determine a manufacturer's corporate average fuel economy (CAFE). However, this proposal would not alter the test procedures, driving cycles measurement techniques, or the calculation methods used to determine CAFE".
We note that USEPA has simultaneously notified the World Trade Organization of this proposed rule so that foreign car makers can take fair notice, and be able to properly label their products for US sales in 2008.
Remember TreeHuggers: the rule, as ust proposed, won't take effect for nearly another two years. Until then, you can use the estimated percentages cited for commonsense adjustments to your "ROI" projections.