Federal regulators are expected to propose a broad revamping of fuel-economy standards that could pressure auto makers to improve the gas mileage of some of their most lucrative sport-utility vehicles.

The National Highway Traffic Safety Administration wants to close loopholes that, for instance, allow DaimlerChrysler AG's PT Cruiser to be classified as a light truck, making it subject to laxer gas-mileage rules than a car. Another loophole the agency is targeting allows massive vehicles such as General Motors Corp.'s Hummer H2 to escape any regulatory limit on fuel economy.

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But certain variations of a weight-based system also could put the Detroit auto makers at a disadvantage. One way would be if the government were to require the same average fuel-economy standard for vehicles above a certain weight, pressuring manufacturers to reduce the weight of their heaviest vehicles. The National Academy of Sciences panel said that lowering the weight of the heaviest vehicles -- which the panel defined as being more than 4,000 pounds -- could improve overall highway safety, by reducing the tendency of these vehicles to cause disproportionate damage in crashes with lighter vehicles.

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The U.S. auto industry, which contributed 10 times as much to Mr. Bush's presidential campaign in 2000 than to his Democratic opponent, is nervous about changing the rules of the game, especially when the outcome is so uncertain. "Any change may not be better, it may just be different," Reginald Modlin, DaimlerChrysler's environmental and energy planning director wrote to the NHTSA earlier this year.

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Environmental groups, some of which advocate increasing auto fuel-economy standards to 40 mpg over 10 years, also oppose the creation of a weight-based system. Those groups say SUVs and other light trucks should be required to meet the same fuel-economy requirements as passenger cars, since so many are used that way. "They haul lattes home from Starbucks, not bales of hay," said Daniel Becker, director of the Sierra Club's Global Warming and Energy Program. SUVs account for about 35% of all light-truck sales.