WASHINGTON - Automakers would have to meet a fuel economy standard of 31.6 miles per gallon for cars and trucks by 2015, under a proposal that federal regulators were set to unveil today.

The standards for vehicles built between 2011 and 2015 were more aggressive than some observers expected, and raised the possibility that the government could require U.S. automakers to meet the 35 mpg target they agreed to last year ahead of the 2020 deadline.

Under the proposal, the National Highway Traffic Safety Administration would raise fuel economy rules to 35.7 mpg for passenger cars and 28.6 mpg for light trucks.

The 31.6 mpg goal assumes the fleet of 2015 resembles what's on the road today; should consumers choose smaller vehicles, the number will be higher.

The fleet of new passenger cars is currently required to meet a 27.5 mpg average, while sport utility vehicles, pickup trucks and vans need to hit a target of 22.5 mpg.

A Transportation Department spokesman declined to comment on the plan.

Automakers and environmental groups will have a chance to comment and suggest changes to the proposal, which the Bush administration wants to put into place before President Bush leaves office.

While the highway administration used the proprietary vehicle plans from automakers, the proposal still could cost the industry tens of billions of dollars.

The scheduled Earth Day announcement comes a day after the average price of gasoline nationwide topped $3.50 a gallon for the first time, on Monday.

The moves follow passage of last year's energy bill, which gave the highway administration wide latitude to set new rules as long as the industry met the 35 mpg goal by 2020.

The auto industry had anticipated that the first increases would be the steepest because Congress has not updated the current 27.5 mpg standard for cars since 1975. Last year, new cars averaged 31.3 mpg, while new trucks averaged 23.1 mpg.

Environmentalist Dan Becker said the proposal was a good start, and was likely to produce two-thirds of the 1.1 million barrels per day of oil savings anticipated by the 35 mpg standard.

"I think you need to give the Bush administration credit," Becker said. "They got one right."

What's not clear is whether the federal standard will be in competition with those set by states such as California, which has attempted to set greenhouse gas limits on vehicles that would result in even tougher mileage standards.

The auto industry is fighting California and other states in a number of lawsuits.

Automakers also have said meeting the 35 mpg standard would require major changes in their vehicles, such as making hybrid-electric systems widespread.

But with gasoline prices as high as they are, consumers already had forced the industry toward more efficient vehicles.