But a copy of the report, embargoed for publication Wednesday, was sent to The New York Times by a member of the E.P.A. communications staff just minutes before the decision was made to delay it until next week. The contents of the report show that loopholes in American fuel economy regulations have allowed automakers to produce cars and trucks that are significantly less fuel-efficient, on average, than they were in the late 1980's.

Releasing the report this week would have been inopportune for the Bush administration, its critics said, because it would have come on the eve of a final vote in Congress on energy legislation six years in the making. The bill, as it stands, largely ignores auto mileage regulations.

The executive summary of the copy of the report obtained by The Times acknowledges that "fuel economy is directly related to energy security," because consumer cars and trucks account for about 40 percent of the nation's oil consumption. But trends highlighted in the report show that carmakers are not making progress in improving fuel economy, and environmentalists say the energy bill will do little to prod them.

"Something's fishy when the Bush administration delays a report showing no improvement in fuel economy until after passage of their energy bill, which fails to improve fuel economy," said Daniel Becker, the Sierra Club's top global warming strategist. "It's disturbing that despite high gas prices, an oil war and growing concern about global warming pollution, most automakers are failing to improve fuel economy."

Eryn Witcher, a spokeswoman for the E.P.A., said the timing of the release of the report had nothing to do with the energy bill deliberations.

"We are committed to sharing our scientific studies with the public in the most comprehensive and understandable format possible," she said. "Issue experts are reviewing the fuel economy data and we look forward to providing a summary of the information next week."

Some of what the report says reaffirms what has long been known. Leaps in engine technology over the last couple of decades have been mostly used to make cars faster, not more fuel-efficient, and the rise of sport utility vehicles and S.U.V.-like pickup trucks has actually sapped efficiency. The average 2004 model car or truck got 20.8 miles per gallon, about 6 percent less than the 22.1 m.p.g. of the average new vehicle sold in the late 1980's, according to the report.

At the same time, while General Motors and the Ford Motor Company are the most common targets of environmental groups, the E.P.A. report shows that several foreign automakers have had the sharpest declines in recent fuel economy performance as they move aggressively into the truck market.

The average 2004 model sold by Nissan, Hyundai and Volkswagen was at least a half-mile a gallon less fuel-efficient than in the previous model year, a sharp drop.

"It's appalling that Nissan, V.W. and Hyundai are accelerating in reverse," Mr. Becker said.

Kyle Bazemore, a Nissan spokesman, said the company's new large pickup truck, the Titan, and new large S.U.V.'s, like the Armada, clearly affected its overall results.

"In '03, we didn't have the Titan and Armada," he said. "We've entered into new markets, but we feel we are doing it responsibly."

John Krafcik, vice president of product development and corporate strategy at Hyundai, pointed out that his company sells relatively few S.U.V.'s but has recently increased its offerings. "Car by car, we're improving fuel economy on every model in our range," he said. "That's a more appropriate way to look at it."

David Friedman, a research director at the Union of Concerned Scientists, an environmental group, disagreed.

"The 8.5 million barrels a day that American cars and trucks use have to do with the vehicles on the road, not the model-by-model comparisons," he said. "What matters to our oil consumption is the fuel economy of the fleet on the road."

Of the eight major automakers examined in the report, only G.M., Toyota and Honda showed increases in fuel efficiency in the 2004 model year, the most recent year for which hard sales data is available. Ford had the lowest mileage of the group. Honda, which does not sell the heaviest kinds of trucks, had the best overall mileage.

Some foreign companies do not even trouble themselves to follow fuel economy regulations. BMW, in fact, has paid more than $70 million in fines since the 2000 model year for noncompliance. The company has argued that American fuel regulations, which are taken as an average over a company's entire fleet, put luxury makers at a disadvantage.

"We sell the ultimate driving machine and people expect us to offer them the kind of BMW vehicles they really want to drive," said Dave Buchko, a spokesman.

Backers of the energy bill have said it will broadly change the nation's energy policy.

Representative Joe L. Barton, the Texas Republican who is chairman of the House Energy and Commerce Committee, said this week that "it is a darn good bill, and it is going to help this country, and the sooner we get it done, the better."

Environmentalists disagree.

"It effectively does nothing to cut our dependence on oil," Mr. Friedman said.

While the proposed bill, as it stands, does offer limited tax credits for hybrid electric cars and advanced diesels, environmental groups object to extending mileage credits for vehicles that can be filled up with an ethanol blend instead of gasoline; many consumers who purchase such vehicles are not even aware of the feature.

The E.P.A. report illustrates what has happened as the industry has poured resources into S.U.V.'s, minivans and family-oriented pickup trucks, vehicle types with less stringent fuel economy requirements than cars. The average new vehicle weight has risen to about 4,000 pounds today, from about 3,200 in the early 1980's. At the same time, the horsepower of an average engine has roughly doubled over two decades, trimming four seconds from the time it takes for the average vehicle to accelerate from zero to 60.