WASHINGTON -- Consumers may be in for a different kind of sticker shock starting with 2008 model cars, trucks, and SUVs.
Estimates of how far vehicles will travel on a gallon of gas are apt to drop by as much as 30 percent -- not because gas mileage is worse but because the testing that determines miles per gallon is getting better.
Congress last year ordered changes in how the Environmental Protection Agency estimates mileage for each vehicle, a response to complaints that the miles per gallon consumers get is often less than advertised.
''The current fuel economy labeling system is broken," said Don MacKenzie of the Union of Concerned Scientists.
Under changes proposed yesterday by the EPA, fuel economy stickers would show most 2008 models getting 10 to 20 percent less gasoline mileage in city driving and 5 to 15 percent less in highway use. Gas-electric hybrids would be affected even more, with ratings for city driving decreasing 20 to 30 percent.
Currently, the EPA relies on data from two tests for the city and highway estimates. They're done in mild conditions, at 75 degrees Fahrenheit, using top highway speeds of 60 miles per hour and average speeds of 48 miles per hour. Those conditions, the EPA acknowledges, are ''generally lower than those experienced by drivers in the real world."
The EPA proposed a new testing regime to develop more accurate mileage estimates from vehicle-specific data.
Automakers starting in 2011 would have to perform extra driving tests to reflect high-speed driving, rapid acceleration, use of air conditioning, and cold temperatures not now part of the agency's lab tests.
The new, lower ratings, however, would not be used to gauge compliance with government regulations requiring automakers to produce fleets averaging at least 27.5 m.p.g. for cars and 21 m.p.g. for light trucks.
EPA administrator Stephen Johnson said the Corporate Average Fuel Economy program, run by the Transportation Department, uses separate requirements for determining fuel economy.
Johnson said the lower figures his agency will develop are intended ''to empower consumers with the most accurate information possible about a vehicle's fuel economy," including more details about the effects of ''power-hungry accessories" used to lower windows, adjust seats, or even play DVDs while driving.
Fred Webber, president of the Alliance of Automobile Manufacturers, said the industry supports the EPA's proposal. But he, like the agency, cautioned that real-world conditions will vary.
''Mileage varies due to weather, road conditions, obeying the posted speed limits, tire inflation, and other vehicle-maintenance conditions," Webber said.
The EPA hasn't revised its fuel economy ratings in 20 years.