In fact, truck manufacturers have not been nearly as resistant to fuel economy standards as car manufacturers have sometimes been. Experts say that if all the manufacturers are pushed into building more efficient trucks, customers will have a reason to buy new vehicles, even if they cost more.

The Environmental Protection Agency said that the rules would cost vehicle buyers $8 billion, but that would be paid for in fuel savings in a year or two, depending on the vehicle. Total benefits, including less time spent refueling, and lower global-warming emissions, would exceed costs by $49 billion over the lifetime of the trucks, the agency said.

While the percentage gains are significant, some of the vehicles go only six miles or so on a gallon, so the actual miles per gallon numbers will not rise much. But the vehicles can cover more than 200,000 miles a year.

The agency may expand its regulations later to cover the aerodynamics of trailers.

Commercial trucks used about 22 billion gallons of diesel fuel last year, a number that experts say could be cut substantially. Heavy-duty vehicles also consumed a significant fraction of the 150 billion gallons of gasoline and ethanol sold last year.

Drew Kodjak, a vehicle expert who served on a National Academy of Sciences panel that studied how large a fuel economy improvement was reasonable, said that no matter what the precise numbers specified in the new rule, simply publishing it was important to lay the groundwork for future improvements. The rule, Mr. Kodjak said, “establishes the key elements of the program,” including the standard course over which the vehicles would be measured, the existing level of fuel economy and determination of who is responsible for improvements. Rules can be tightened over time, he said.