The National Highway Traffic Safety Administration is about to rule on the federal fuel economy standard for new-car fleets, perhaps this week, according to auto industry and federal sources.

The betting is that the Reagan administration will heed a request from General Motors Corp. and Ford Motor Co. to lower the standard from the current 27.5 miles per gallon to 26 mpg.

Under a 1975 law, all automakers selling cars in the United States are required to meet average "maximum feasible" fuel economy levels for their fleets -- the corporate average fuel economy (CAFE).

GM and Ford, the nation's two largest car companies, make big profits selling big cars. But big cars also pull down their CAFE ratings.

GM, for example, is running 2.4 mpg short of meeting the current CAFE standard and Ford is 1.6 mpg below it. Both companies contend that they have tried to live within the law but can't because customers continue to prefer full-size, rear-wheel-drive cars that tend to use more gasoline over smaller, front-wheel-drive models.

The situation seems particularly painful for GM. The company spent billions of dollars developing a brand new line of "downsized," luxury front-wheel-drive models, such as the Oldsmobile Ninety-Eight Regency, the Buick Electra and Park Avenue, and the Cadillac Sedan De Ville and Fleetwood. But customers have been leaving those on the showroom floor in favor of the larger, rear-wheel-drive versions.

GM, as a result, is offering a consumer incentive -- 9.9 percent retail financing from June 21 to July 31 -- to increase sales of the new line.

Reagan administration officials in recent months have expressed sympathy for GM's plight and have indicated that they are willing to go along with the automaker's argument that fuel economy standards should be set in the marketplace, instead of in NHTSA's offices.

"The administration holds that free-market forces, rather than statutory standards, are the most efficient means of achieving appropriate levels of fuel economy in the long run," James H. Burnley IV, deputy secretary of transportation, said at a June 4 meeting of the Motor Vehicle Manufacturers Association of the United States Inc.

Officials at the Commerce and Energy departments have made similar statements. NHTSA officials have said that such comments will not determine their final decision. But few of them accept the notion that the ruling will be fashioned by NHTSA Administrator Diane K. Steed, or by NHTSA alone.

The law states that the CAFE must be determined before the start of the new-car model year that is involved. New cars usually start rolling off the lines by early August for introduction in mid-September. "They're under an awful lot of pressure at NHTSA" to rule soon, one Capitol Hill source said.

Chrysler Corp. has been the loudest auto industry critic of the proposal. Chrysler's profits come mainly from small and mid-size cars.