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 dropping by as much as 30 percent -- not because gas mileage is getting worse but because the testing that determines miles per gallon is getting better, regulators say.

Congress last year ordered changes in how the Environmental Protection Agency estimates mpg for each vehicle, a response to complaints that the mpg they get is often less than advertised.

"The current fuel economy labeling system is broken," said Don MacKenzie, a vehicles engineer with the Union of Concerned Scientists.

Under testing changes proposed Tuesday by EPA, fuel economy stickers will show most 2008 models getting 10 percent to 20 percent less gasoline mileage in city driving and 5 percent to 15 percent less in highway use. Gasoline-electric hybrids will be affected even more, with ratings for city driving decreasing an average of 20 percent to 30 percent.

Currently, EPA relies on data from two lab tests for the city and highway estimates. They're done in mild conditions, at 75 degrees Fahrenheit, using top highway speeds of 60 mph and average speeds of 48 mph. Those conditions, EPA acknowledges, are "generally lower than those experienced by drivers in the real world."

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 laboratory tests.

The new, lower ratings, however, will not be used to gauge compliance with government regulations requiring automakers to produce fleets averaging at least 27.5 mpg for cars and 21 mpg 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, even play DVDs while driving.

"They can be confident those estimates more closely reflect real-world conditions," he said.

Fred Webber, president of the Alliance of Automobile Manufacturers, said the industry supports EPA's proposal and will help the agency adjust the new vehicle window stickers. But he, like the agency, cautioned that real-world conditions will vary from driver to driver.

"Mileage varies due to weather, road conditions, obeying the posted speed limits, tire inflation and other vehicle-maintenance conditions," Webber said.

Others issuing statements of support included Ford Motor Co. and the American Automobile Association, which joined EPA in making the announcement. "Consumers want to know that the information they see on a government-sanctioned label reasonably reflects what they will experience on the road," said AAA president Robert Darbelnet.

EPA hasn't revised its fuel economy ratings in 20 years. The agency also plans to redesign the stickers so they are more consumer-friendly.

"It's obvious that the driving world has changed a lot since 1985," said Johnson, who noted that 20 years ago he proudly drove a full-size coupe Pontiac Catalina back and forth to work. "My car really would not match up to today's vehicles."

Bluewater Network, which petitioned EPA to change its testing procedures in 2002, mostly welcomed the changes. "This has been a long time coming," said Danielle Fugere, the group's global warming campaign director.

But Dan Becker, director of the Sierra Club's global warming program, said that because the rules for complying with the CAFE standards aren't changing, "the bottom line here is that this will have zero effect on oil savings or environmental impact."

Source: Associated Press