LARGELY BECAUSE of the fleet average mileage standards enacted by Congress five years ago, new American-made cars are getting an EPA test mileage of 21.5 mpg. But the fleet average of new imports is about 30.5 mpg, and with gas at $1.50 per gallon, the difference in average yearly gas costs is between $200 and $300. The result is that the U.S. auto industry is steadily losing ground to the foreign makers -- hurting American workers, increasing the trade deficit and helping to unbalance the federal budget, which must cover Detroit's growing unempolyment costs.

The question now is whether additional standards are needed when the current ones expire in 1985. Is an average of 27.5 mpg -- the 1985 standard -- all that is technologically and economically possible?

Today there are few experts who doubt that much higher fuel economies can be achieved -- and at an acceptable price. Volkswagen, for instance, is already testing a four-passenger car that has an EPA rating of 70 to 80 mpg . Detroit, on the other hand, still seems to be looking in the other directon: Pontiac has on its drawing boards a two-passenger "commuter" car that is much heavier and has a larger engine than the existing four-passenger VW Rabbit.

But are new government standards really needed -- or will rising gas prices and consumer demand for more efficient cars persuade Detroit to make high-mileage cars? The answer seems pretty clear: standards will bring more improvement at a lower cost to the industry and much sooner than would occur without them. Instead of continuing to lag several years behind consumer demand and economic realities, standards would give Detroit a long-range goal that could be planned for with a high degree of certainty and economic efficiency.

The one serious argument against new standards is their cost. Detroit may not be able to afford the meassive capital investments that will be required, and assistance from the government may therefore be necessary -- probably between $50 billion and $100 billion over a 10-year period. That is a mountain of money. But the alternatives are at least as costly. For example. The administration plans to spend $88 billion for a synthetic fuels industry that might produce about two million barrels of fuel per day. More efficient cars could save more than that amount -- and with environmental benefits rather than damage. Detroit's mayor, Coleman Young, put it nicely: "Do we really want to run Japanese-made cars in this country or American-made synthetic fuel?"

If the country's 100 million automobiles had an average fuel economy of 40 mpg instead of their current 15 mpg, and if its 30 million vans and pickups (which are now exempted from the mileage standards) were getting 30 mpg instead of 12 mpg, U. S. oil needs would be cut by four million barrels a day. That is enough to cut current imports in half, and is considerably more than the country now imports from all the Middle Eastern producing nations combined. The real benefits this change would bring to the national security are to obvious to bear repeating.

Sen. Henry Jackson and Warren Magnuson have taken the first step in the right direction with a bill that would raise the country's new-car fuel-economy standards to 40 mpg in 1995. Until a few days ago, their proposal seemed momentous -- but maybe too bold to win wide support in Congress. Then a report appeared of a soon-to-be-finished government study that recommends that the fleet average standards for 1995 be raised to 85 mpg. All of a sudden, 40 mpg sounds like a modest goal.