GASOLINE and diesel engines consumed just under 90 percent of all the oil produced in the United States last year. Incredibly, American cars consume one-ninth of all the oil used in the world on any given day. That is why the mandatory fuel economy standards enacted in 1975 are of such central importance to the nation's energy balance. Now, however, it seems that, although it is technically feasible to do so, the efficiency standards are not being met.

After seemingly endless debate and strong opposition from Detroit, the 1975 law required that the fleet average efficiency for each manufacturer in 1985, as measured in tests performed by the Environmental Protection Agency, be 27.5 mpg -- double what it had been at the time of the embargo. However, a recent study by the Department of Energy revealed what every driver already knows -- that the EPA ratings do not measure what the car will actually do in normal driving. In fact, these tests overstate fuel efficiency by 20 percent to 28 percent.If the discrepancy is allowed to persist, the 27.5 mpg standard for 1985 will actually reflect an efficiency of less than 22 mpg -- a lost savings of more than 1 million barrels of oil per day.

A recent article in Science magazine describes the sources of this difference between EPA ratings and actual performance. About half of the difference arises because the 1975 law required that each year's ratings be based on a set of idealized assumptions that were in use at that time -- when there was little accumulated experience with this kind of testing. These assumptions include, for example, that all driving is done on warm, clear days and on straight and level roads. The purpose was simply to make each year's figures comparable with the next year's; the result has been to freeze these distortions into the system. The other half of the difference comes problems with the testing procedures. For example, EPA has found that some of the cars it tests are "hand-built" prototypes that differ a good deal from the cars that later roll off the assembly lines. Also, to make better efficiency grades, automakers were using a gearshift technique that no ordinary driver would ever use.

Congress and the administration need to take a hard look at the implementation of the 1975 standards. Perhaps the law needs to be changed to make the tests more realistic. Perhaps more effort needs to be put into enforcement. Whatever needs to be done should be, for no other single measure will save as much energy as quickly as will the production of more efficient cars and trucks. There is a lot of room for improvement. This country, which justifiably prides itself on its technological strengths, did not place one car in or even near the top 10 of the 1980 fuel efficiency ratings announced a few days ago. There were four Chrysler models in that group -- all made in Japan.