The Reagan administration is studying the desirability of trying to repeal or reduce the federal minimum mileage standard for new cars to avoid having to levy hundreds of millions of dollars in fines against the nation's two largest automakers.

With gasoline prices falling over the past year, General Motors Corp. and Ford Motor Co. have been selling more big cars and fewer compacts and diesels. This has reduced the fuel economy average for all of the cars that these two companies manufacture below 26 miles per gallon, the level required for the 1983 model year.

Although it appears that any 1983 penalties largely would be offset by credits from past years when the automakers surpassed the mileage standards, GM and Ford face the possibility of huge fines when the average rises to 27 mpg for the 1984 model year and 27 1/2 mpg for the 1985 model year.

The law calls for a fine of $5 per tenth of a mile per gallon per car for all automobiles produced by a company not in compliance with the standard. GM and Ford manufactured more than 5.5 million automobiles in 1982.

While federal officials say the magnitude of the problem won't be clear for a few more months, Ford told the government that it anticipates an all-model average for 1983 of about 24.3 mpg. GM's average is expected to be even lower.

Some auto industry analysts, however, feel that the problem faced by GM and Ford is the result of an aberration in the marketplace, and that the inevitability of higher gasoline prices may make it unnecessary for the government to take any action.

The President's Council of Economic Advisers recently concluded a study of the implications of this problem, and Christopher DeMuth, top regulatory policy maker in the Office of Management and Budget, will meet with Transportation and Commerce Department officials in a week to discuss how to deal with it.

One possibility is seeking repeal of the law, enacted to spur the development of fuel-efficient cars following the 1973 Arab oil embargo.

Any such move, however, would be expected to run into major opposition on Capitol Hill, and to bring cries of protest from Chrysler Corp., which has moved more aggressively than GM or Ford to comply with the law. Chrysler, which sells a larger proportion of small cars than GM or Ford, expects to reach the 1985 average of 27 1/2 this year.

A second possibility is capping the mileage requirement at 26, and not letting it rise to 27 1/2 over the next two years.

"The National Highway Traffic Safety Administration has the authority to reduce the requirement from 27 1/2 to 26 mpg and even below that," said Dick Burdette, NHTSA's director of public and consumer affairs. "But if we were to try to set a requirement below 26, it would be subject to a one-house-of-Congress veto."