The test procedure is being changed for a lot of reasons, but mainly it's because the government has realized that people don't drive much like the computer model used in today's testing.
They are going to adjust the procedure to reflect our higher freeway speeds, the fact that people tend to accelerate much harder than the computer, our constant use of air conditioning and the effects of adverse-weather driving. We can only guess that most estimates will go down somewhat in this new formula. And we are not alone.
Scared at the possible backlash when their vehicles appear to be getting poorer mileage, major carmakers have banded together to create a website, www.mileagewillvary.com, to help educate consumers about the apparent drop in fuel economy.
Consumers tend to be pretty savvy and at times ignore the estimates altogether, so we expect most people will adapt pretty quickly. One area that may be a concern, however, is the CAFE requirements, which may or may not be altered to reflect the new measuring procedures.
[Source: Automotive News, sub req]