From http://cascadiascorecard.typepad.com/bl ... index.html
With gas prices soaring, some people may trade in their gas-guzzlers for more fuel efficient vehicles. But don't trust the EPA ratings. A recent analysis by Consumer Reports shows that 90% of vehicles get worse gas mileage than advertised -- in some cases more than 50% worse for city driving. And nationally, Corporate Average Fuel Economy (CAFE) may be overstated by a whopping 30 percent.
How does this happen? Manufacturers inflate fuel efficiency in several ways. First, they don't test cars the way people actually drive. Vehicles are tested in a laboratory, not on actual roads. And while the EPA assumes that 55% of driving is done in city traffic, which uses more fuel than highway driving, many cars actually spend 62% of time there, according to Consumer Reports. Second, the car they test is not the car you buy. Manufacturers are allowed to use prototypes built especially for the fuel economy test, so they often modify them (within limits) to get the best rating possible.
In 1984 the EPA responded to an outcry by consumers who were angry that they could not get the fuel efficiency advertised. But rather than change the way it tests cars, the EPA just adjusted the test results it reports: 10% lower mpg for city driving, 22% lower for highway. And, as Consumer Reports shows, even these adjusted numbers still aren't accurate, especially in city driving.
And worse, CAFE standards, already low and full of loopholes, are also affected. Automakers successfully lobbied so that only the unadjusted mpg ratings are used when enforcing CAFE standards. While the government estimates that the fleet of 2003-model-year passenger cars that Consumer Reports tested averaged 29.7 mpg, Consumer Reports only got 22.7, well below the current standard of 27.5 mpg. For light truck the difference was 21.4 mpg versus 16, with a standard of 20.7 mpg.
When buying a new car, follow Consumer Reports's advice:
The EPA sticker can help you evaluate relative gas mileage among vehicles, but not absolute mpg.. .. [D]iscount the EPA sticker numbers for city travel as follows: conventional cars and trucks, 30 percent; larger hybrids, 35 percent; diesels, 36 percent; smaller hybrids, 42 percent.