WASHINGTON — The Bush administration will unveil landmark changes to federal fuel economy standards for light trucks this week, the first major overhaul since Congress mandated them for all vehicles in 1975 after an Arab oil embargo sent gasoline prices soaring.

The new fuel economy rules covering pickups, sport utility vehicles and minivans could be a watershed event for the auto industry, which has strained to meet consumer desire for bigger and more powerful vehicles while still meeting the government's minimum fuel economy targets.

With gas prices reaching all-time highs, the regulations are expected to renew a debate over the best way to achieve more miles per gallon without harming automakers or reducing passenger safety.

Detroit automakers believe the new system will provide them with flexibility they don't have under existing rules that set an overall mileage requirement for the entire fleet of light trucks they sell.

The National Highway Traffic Safety Administration is expected to establish different fuel economy requirements based on a light truck's size. That is expected to make it easier for General Motors Corp., Ford Motor Co. and DaimlerChrysler AG's Chrysler to sell bigger pickups and SUVs — a key source of profits — without penalty.

Automakers are required to maintain an average of 27.5 mpg for cars and 21.0 mpg for light trucks, which include pickups, SUVs and minivans.

Detroit's Big Three argue that current rules reward companies like Honda Motor Co. and Hyundai Motor Corp., which primarily sell smaller SUVs and minivans while amassing credits that can be used to offset sales of bigger vehicles.

Larger pickups and SUVs, such as the Ford F-series and the Chevrolet Suburban, have been a profit stronghold for Detroit. GM, Ford and Chrysler say the current Corporate Average Fuel Economy program puts them at a disadvantage because sales of the biggest gas-guzzlers need to be offset by smaller, more efficient models.

"On a model-to-model basis, we know our models stack up well against the competition," said GM spokesman Chris Preuss. "Yet if you look at the CAFE numbers, it doesn't appear we're doing as well."

Environmental groups believe the new rules will do little to meet the primary goal of the fuel economy program — saving oil.

The Bush administration has made improved fuel economy a major initiative to address rising gas prices, national security concerns about foreign oil imports and the link between auto emissions and global warming. In 2003, the administration proposed a modest increase in light-truck fuel economy, from 20.7 mpg in the 2004 model year to 22.2 mpg in the 2007 model year. The new rules will take effect with 2008 models.

The fuel economy rules are the industry's single most expensive regulatory obligation, and Detroit automakers say any stiff new requirements could further strain the industry. In North America, GM and Ford are already coping with major financial losses.

In the official notice that it would overhaul Corporate Average Fuel Economy rules, NHTSA said it would look only at light trucks because it is required by law to set a fleet average for light trucks every year. Officials considered adjusting rules covering cars, but the legal authority for the agency to do so was unclear.

NHTSA has also emphasized safety in revising the rules. The agency's emphasis on standards for different sizes of vehicles is a response, in part, to a 2001 study by the National Academy of Sciences that said a size-based system might save lives by reducing the number of crashes between mismatched vehicles. Congressional efforts to raise fuel economy standards have failed in part because of fears automakers would sell lighter vehicles to improve mileage, making them less sturdy in crashes.

Environmental groups are disappointed the new rules will not target passenger cars, which have fallen out of favor with many consumers but still account for 44 percent of industry sales.

"The big question is: With this new structure, will they save oil?" said David Doniger, senior attorney of the Natural Resources Defense Council. "If they change the structure, and they don't save oil, then all you're doing is rearranging the deck chairs on the Titanic."

On Friday, the Union of Concerned Scientists released a report outlining gaps in the fuel economy rules, including provisions that allow car-like vehicles such as the Chrysler PT Cruiser to be counted as trucks. NHTSA also excludes trucks that weigh more than 8,500 pounds and gives fuel-economy credits to companies for selling "flexible fuel" vehicles. The loopholes have undercut the fuel economy program by 1 million barrels of oil a day, the group said.

NHTSA is expected to retain the exemption from fuel economy requirements for trucks weighing more than 8,500 pounds, such as the Hummer H2.

Today's cars get double the mileage of equivalent models in the 1970s, said Eron Shosteck, spokesman for the Alliance of Automobile Manufacturers. While fuel economy of the average vehicle has remained flat since the late 1980s, Shosteck said today's cars have bigger, more powerful engines without losing fuel economy.

"Automakers can do one part of the equation: They provide the vehicles," Shosteck said. "They don't have control over the other part, what people buy and how they drive."