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CHICAGO _ Your mileage may vary.
Those four words have been linked to the EPA's fuel-economy estimates since the 1974 model year, when automakers began disclosing fuel efficiency of their cars and light trucks on window stickers in the wake of the OPEC oil embargo.
When the EPA issues its fuel-economy estimates for 2003 vehicles in October (www.fueleconomy.gov), critics say those words will be more of a promise than a warning because the numbers are based on assumptions that no longer apply to the way most Americans drive.
The EPA has not changed the way it calculates fuel economy since 1985, when it reduced mileage estimates generated in laboratory tests by "correction factors" of 10 percent for city driving and 22 percent for highway driving to make the numbers closer to what motorists achieve in the real world.
In the 17 years since, the number of vehicles on U.S. roads has increased 41 percent, to 221 million, according to the Polk Co. The Federal Highway Administration said the number of miles driven per year has grown 57 percent, to 2.78 trillion.
With so many more vehicles, logging more miles, traffic has slowed to a crawl on many congested city streets.
The EPA, however, still bases its city estimates on a simulated driving cycle developed nearly 30 years ago that averages 20 miles per hour, a pace many drivers only fantasize about.
When the EPA created its highway mileage test, which averages 48 mph, …