An industry-backed study by The Mellon Institute recommends that the nation meet its long-range needs for motor fuels by speeding up synthetic fuels developed rather than pushing for further improvement in car and truck mileage beyond the current goal of 27.5 miles per gallon by 1985.

The study, released yesterday, estimated that it would cost $103 billion in today's prices to assure a sufficient supply of motor fuels between now and the year 2000. The cost was calculated to be the same whichever strategy is emphasized, synthetic fuels development or improved mileage.

But the Mellon report says that further investments in redesigning automobiles to improve mileage significantly are no longer cost-effective after the early 1990s. At that point, it is more economical to increase investments in synthetic fuels production, the study concludes.

The study estimates that demand for car and truck fuel will grow from 7.5 million barrels a day in 1979 to 9.4 million barrels a day by the year 2000. Although the fuel supply produced from oil is expected to drop to 3.3 million barrels a day by 2000, synthetic fuel production could add an additional 2.1 million barrels a day, and improved vehicle fuel economy could add 4 million barrels a day, under the approach recommended by the study, the report said.

U.S. auto manufacturers, who formerly insisted the 1985 standard of 2.75 miles per gallon couldn't be met, now believe it can because of the shift toward smaller cars.

After consulting with auto producers, the Mellon study authors estimated that the average new car fuel economy can reach 32.2 miles per gallon in 1985, 37.9 mpg in 1990 and 47 mpg in 2000, based on design and engineering improvements now being planned.

But the study concludes that trying to attain the higher, post-1990 standard is not the cheapest way of meeting the nation's vehicle transportation requirements.