Government researchers are working on fuel efficiency proposals that would force automobile manufacturers to build cars that will average 40 to 50 miles per gallon by the 1990s, according to sources in the Department of Transportation.

The stringent fuel proposals are part of a package of alternative standards Transportation Secretary Neil Goldschmidt soon plans to present to the White House. The 40-to-50 miles per gallon standard is the mileage rate Transportation Department researchers believe is achievable by the mid-1990s, government sources said.

One approach being considered would require the auto industry to increase fuel efficiency by 1.5 miles a gallon a year until the new goal is achieved. "That seems as logical as anything," one high official said yesterday.

Goldschmidt will travel to Detroit in less than two weeks to meet with the heads of the major domestic auto companies, department sources say, and one of the topics of discussion at those meetings is expected to be future fuel economy mandates.

After that trip, Goldschmidt is also expected to meet with his staff researchers to discuss how tough future fuel economy standards should be, and how and when his agency should begin to push for their implementation.

Earlier this week, Deputy Energy Secretary John Sawhill told a congressional subcommittee that "because of this critical role that the automobile plays in our energy picture, it is important that we continue to explore ways to improve mileage efficiency."

Sawhill said exploration will "inclusde an evaluation of higher fuel economy standards for passenger cars beyound 1985 (and) increasingly stirngent standards for light trucks for 1982 and later years."

At a special DOT conference in Boston last April, auto industry techicians, academics and government officials agreed that a 50-mpg car was a reasonable goal for the 1990s, but at an extremely high price.

And at the first internation automotive fuel economy research conference, held this past week under DOT sponsorship in Washington, there was considerable talk about a Volkswagen paper which said that an 80-mpg car was possible, even under present technological limitations.

That paper, written by highly respected Volkswagen engineer Dr. Ullrich Seiffer and titled "Improvements in Auto Fuel Economy," said a typothetical vehicle, equipped with what he called an "optimized diesel engine with optimized transmission," weighing about 2,000 pounds, could achieve 80 mpg and at the same time stay within all federal safety and emission requirements.

At that same conference, National Highway Traffic Safey Administration head Joan Claybrook called on the federal government to launch an "enhanced research program to fully support possible fuel economy rule-makings for the late 1980s and 1990s."

She noted the administration already has forwarded to Congress proposed legislation that would earmark $200 million over the next decade to "support research and assessment of motor vehicle subsystems that will determine to a large degree fuel economy levels beyond 1985, which is the last year for which fuel economy standards have been set."

Claybrook said in an interview that the Boston conference and other discussions also have highlighted the need for more fundamental basic research into such things as the combustion engine, drivetrain components and systems and structural designs.

There has been concern, she said, that the auto industry has neglected such basic research for too long, prefering instead to develop more short term improvements in automobile design.