THE TWO BIGGEST American automobile companies, General Motors and Ford, are asking the government to relax its fuel standard for the coming year. The current standard requires each company's cars -- large and small taken together -- to average 27.5 miles per gallon. Neither GM nor Ford is likely to meet it this year.

The chief reason is that the price of gasoline has been falling for four years and the market is swinging back toward big cars. And there's another, more recent, reason: the Reagan administration's decision to allow rising imports of Japanese cars means that the American companies will sell fewer of their small models than they had expected.

Under present law the secretary of transportation, Elizabeth Dole, has the authority to drop the required average to 26 miles per gallon, and that's what GM and Ford are asking. If they don't get it, they will have to conform next year by raising the prices of their bigger cars high enough to force their customers to choose small ones.

The industry is not unanimous. Chrysler is vigorously opposing any relaxation. Since it no longer makes big cars and will easily meet the present standard, it indignantly asks why it should now be put at a disadvantage in favor of those who have not complied. That's not a trivial objection.

But neither does it settle the case. The purpose of the gasoline efficiency standard is to save fuel and reduce American dependence on imported oil. The reality is that the Reagan administration is not prepared to lift a finger in support of that purpose and, with imports currently declining again, Congress itself is divided. The administration simply says: let the market settle it.

The market? The same oil market that twice in the past dozen years has shot upward and then slowly sunk donward? The market that has twice lurched toward much smaller and more efficient cars, and then each time slowly begun to slide back toward bigger and more powerful ones? The market has its virtues, but in oil and automobiles it is radically unstable and a terrible forecaster. These sudden unpredictable swings have severely whipsawed the auto companies, and have proved immensely costly to the country as a whole.

The way to stabilize both oil demand and the auto market, pushing both toward conservation, is to impose a steadily rising gasoline tax. But since neither President Reagan nor Congress has the slightest intention of doing anything like that, Secretary Dole would be justified -- as a very second- best expedient -- in reducing the standard by mile-and-a-half per gallon. It's unfair to require an industry to bear the full burden of a public policy that the president himself declines to support.