American car makers produce a fleet of vehicles averaging 45 or 50 miles per gallon by 1995 if the federal government required them to do so, says a study completed in the final days of the Carter administration but never made public.

Setting new federal mileage standards for the decade after 1985, when present federal fuel economy mandates expire, could cut the nation's oil imports by at least 20 percent from current levels, according to the report by the National Highway Traffic Safety Administration.

Former secretary of Transportation Neil Goldschmidt refused to release the report, and Drew Lewis, Ronald Reagan's transport chief, also is expected to reject the recommendation. A copy of the NHTSA study was obtained by The Washington Post.

Goldschmidt, the Reagan transition team and auto company executives insist there is no need for additional federal regulation of automobile fuel economy. Market forces -- including rising fuel prices and increased public awareness of energy issues -- will lead the industry to improve its gas mileage, they argue.

The NHTSA study disputes both contentions. It warns that complacency by consumers and reluctance by car makers could frustrate efforts to maximize fuel economy and force the nation to import millions of barrels of additional oil.

The debate could set up an early confrontation between lingering liberals who believe in regulation and up-and-coming conservatives opposed in principal to any government interference in the marketplace.

Regardless of whether more efficient cars are built by government fiat or industry initiative, there are more than a dozen ways to improve gas mileage using existing technology, says the NHTSA report and another study released last month by the Congressional Budget Office.

Since 1975 the average fuel utilization of domestic cars has increased from 14.8 miles per gallon to 21 mpg. American car makers, who were complaining less than two years ago that they could not meet a government mandate of 27 1/2 mpg by 1985, now say their fleets will average 31 mpg by then.

Present law does not increase the minimum mileage standard after 1985; NHTSA on Monday filed a formal request for comment on post-1985 fuel economy standards.

Without endorsing any future regulation, the CBO last month concluded that "existing technologies coupled with market shifts to smaller cars, could bring the average new fleet fuel economy to about 37 to 42 mpg by 1995."

The NHTSA report says even more miserly cars are possible: "The domestic manufacturers probably can have the technological capability to increase average fuel economy of passenger autos by 1995 to the neighborhood of 45 to 50 mpg."

Ironically the NHTSA study utilizes one of the conservatives' favorite techniques, cost-benefit analysis, to justify new fuel economy regulations.

The NHTSA analysis says the present federal gas mileage rules save the average buyer of a 1980 car $1,700 in gasoline (at $1.25 a gallon) and will produce an additional saving of $1,600 for buyers of 1985 cars, even if gasoline prices do not increase.

The CBO report estimates it will cost $600 to $650 to further improve passenger car fuel economy. But over 100,000 miles, the more efficient cars will save $1,100 worth of gas.The benefit exceeds the cost by about $500, the CBO concluded.

But the bills for increased fuel economy first will have to be paid by the automakers, and the CBO estimates they will have to invest between $8 billion and $12 billion a year to produce 40-mpg cars by the 1990s.

The industry spends about $7 billion a year for "business-as-usual" capital requirements and will have to come up with an extra $1 billion to $5 billion annually. Such massive expenditures are "likely to continue to strain the industry's financial capacity," the agency warns.

But if Detroit fails to spend the money necessary to make the most efficient cars possible, foreign car makers will continue to take away a massive share of the business by offering consumers cars that get better gas mileage.

And foreign oil producers will continue to drain billions of dollars a year out of the U.S. economy. Pushing for a 50-mpg fleet average could save the country 1.1 billion barrels of oil a day, the NHTSA study estimates.

The way to reduce the amount of gasoline and diesel fuel burned by passenger cars is not to force everyone to trade in big cars for little ones, but to improve the operating efficiency of present-size cars, the NHTSA suggests.

Only a 1.2-mpg gain in average gas mileage will result from a massive shift to smaller cars in the next five years, NHTSA researchers said.

Getting to a 45-mpg or 50-mpg fleet average by the 1990s will require paring the weight of the average car by another 350 pounds, installing diesel engines in 35 percent to 40 percent of all autos and pursuing several other avenues to efficiency, the study says. None of the changes requires any technological breakthrough, but many of them demand widespread utilization of little-used innovations.

Mostly, however, the prescription is to continue existing auto industry trends: more downsizing, more front-wheel drive, more diesels, more electronic engine controls, etc.

About 60 percent of U.S. cars produced in 1985 will be the result of two full cycles of downsizing, the study says, and another 20 percent of the market could be downsized again, producing a 10 percent gain in fuel economy at a cost of $146 a car.

Front-wheel drive will be used in 80 percent of American cars by 1985. Switching the rest of the fleet to FWD will boost average fuel economy by 12 percent at a cost of $166 a car.

Computerized engine controls that regulate more carefully the amount of fuel consumed are just showing up in cars and will be used in only 5 percent of the 1985 models, the CBO projects. Putting electronic controls in a quarter of the cars would yield a 15 percent boost in fuel economy at a cost of $179 per car.

Another 15 percent gain could be produced if diesel engines are used in 30 percent of cars by 1995. Diesels are the most expensive single technique for improving fuel consumption, costing a projected $679 a car. Optional diesels from General Motors Corp. and Volkswagen now cost less than that, but the industry will have to design a new family of diesel engines with a massive investment in tooling.

A turbocharger -- an air compressor that forces more fuel and air into the engine -- can produce a 10 percent boost in fleet fuel economy if they are used on 30 percent of the new models. Turbos are expensive (CBO estimated $332 per car), they add to maintenance costs and cars equipped with them get worse gas mileage than cars with the same size normal engine. But with a turbo, smaller engines can be used, producing some fuel savings.

Increased use of plastic, aluminum and other light-weight materials can produce a 4 percent increase in fuel economy at a cost of $131 per car. Adding energy-efficient air conditioners, power steering units and engine fans can contribute another 2 percent to mileage for about $16 per car.

Another 5 percent gain could come from better streamlining, which should cost no more than $17 a car if the vehicle is being redesigned anyway. Tires that roll easier and bearings with less friction could add 2 percent to mileage for about $26 a car. An estimated 3 percent gain is projected by switching to smoother oils in engines for further reduction in friction at an estimated cost $1.25 to $15 per car.

Manual transmissions get better gas mileage than automatics, but most Americans prefer not to shift for themselves. The CBO and NHTSA suggest that the solution is installation of a four-speed automatic with a torque converter lock-up device. Ford and GM already offer such transmissions on some 1981 models. Their use in 54 percent of 1995 models could produce an overall 6 percent gain in fuel economy for about $198 more a car.

The main difference between CBO projections that these changes could produce a fleet of 40-mpg cars by 1995 and NHTSA's projected 45- to 50-mpg average is the separate estimates on how fast the innovations can be adopted.

The depends on how much the auto industry can afford to invest in new plans, plants and equipment. Both government agencies contend fuel economy mandates.

GM officials have claimed that each one-half-mile-per-gallon in additional fuel economy requires a $1 billion investment. Since 1974, GM's corporate average fuel economy has improved from 12 mpg to 21 mpg; using the company's numbers, the fuel innovations alone would have required an $18.8 billion investment. But GM invested only $20.7 billion in capital spending during the period, the CBO noted, and long before fuel economy expenditures began, the company was spending a couple of billion dollars every year on annual model changes and other capital costs.

Other automakers also have blamed government regulation for expenditures they would have made anyway, the CBO said: "The industry's entire capital spending program (both foreign and domestic) has been attributed to fuel economy."