The compromise should ensure passage in the House, although the Senate may insist on changes. It does not appear to include provisions, like $16 billion in new taxes on the oil industry, that drew a veto threat from President Bush.

The latest version of the measure, if it becomes law, will force wrenching changes on the American car companies, from design studios to new-car showrooms to executive suites. Automakers now have to achieve 27.5 miles per gallon on cars, a figure that has not changed since 1984, and 22.2 miles per gallon for light trucks, including minivans, sport utility vehicles and pickups. Under the compromise, the companies will retain the distinction between the classes of vehicles, but must still meet a combined 35 m.p.g. fleetwide standard.

Ms. Pelosi called the compromise on mileage “an historic advancement in our efforts in the Congress to address our energy security and laying strong groundwork for climate legislation next year.” She said that she was confident it would win the backing of environmentalists, auto makers and labor and would clear Congress by the end of this year.

Mr. Dingell, in a statement, called the new mileage standard “aggressive and attainable.”

“After weeks of productive discussion and negotiation, we have achieved consensus on several provisions that provide critical environmental safeguards without jeopardizing American jobs,” he said. Critical to his agreement, he said, were incentives to the American auto industry for producing small cars in the United States and cars that run on a combination of gasoline and ethanol.

The Big Three automakers have warned that complying with the new fuel economy rules will cost them tens of billions of dollars and rob consumers of choices. But even if they meet the law’s mandate, the fuel efficiency of the American car fleet will still lag far behind that of other major industrialized countries.