A FEW MONTHS AGO, Scientific American published an analysis of the future American market for cars and light trucks.The authors, Charles Gray and Frank von Hippel, concluded that the average fuel economy of the whole U.S. fleet built in 1995 could be 60 miles per gallon -- even allowing for 40 percent of the sales to be of large cars and light trucks.

According to their calculations, which were based on improvements of existing technologies, the costs of the necessary changes would be less than $1 per gallon of saved fuel. Considering that the comparable fuel efficiency figure for 1980 -- that is, the average on-the-road fuel economy of new cars and trucks -- was 18 mpg, the conclusion seemed somewhat incredible, more an academic exercise than a projection of practical reality.

Now, however, comes the announcement that Volkswagen is planning full-scale production of a four-passenger car that will deliver 75 mpg or more. The car will appear on the market, the company says, not in the distant 1990s, but in three or four years. Auto industry sources report that prototypes have scored well above 90 mpg on the EPA mileage test. The car has a three-cylinder, supercharged diesel engine with an energy-storing device that shuts off the engine when it is declerating or stopped, then automatically restarts it. There is also a new type of transmission that increases engine efficiency by delivering more of its power to the car's wheels.

What would owning a 75-mpg car be like? Well, if you drove your car about 150 miles a week, it would mean a visit to the gas station to fill your 10-gallon tank about once every five weeks. Whether or not the VW car eventually does all that the company claims for it, the announcement of the prototype test results are proof that really high mileage cars are no longer the stuff of sci-fi dreams.

The key question is whether the American auto industry, still an essential component of a healthy national economy, has made the correct bet in planning to introduce 40- to 50-mpg cars in the same period. Has the industry accurately predicted consumer demand, or has it underestimated -- as it did in the 1970s -- Americans' desire for high fuel efficiency? Having spent tens of billions on retooling in order to build cars in the 20- to 40-mpg range, will Detroit be leapfrogged again by another generation of foreign technology.