If the nation is willing to pay the price, U.S. auto companies can manufacture fleets averaging 50 miles a gallon within the next 20 years, a blue ribbon panel of auto engineers, research scientists and government regulators concluded.

That was the major news emerging from a two-day conference called by Transportation Secretary Brock Adams to bring together the auto industry and the government in an unprecedented attempt to pool resources to develop the safest, most fuel efficient and practical car of the future.

But the real news may be the erosion of distrust and antagonism between the government and the industry. The two sides long have been at odds over the costs and benefits of federal regulation. And the auto industry in recent months has intensified its attack on safety and fuel efficiency regulations which it claims are overly inflationary.

There appeared to be a spirit of cooperation at this conference, however, with regulators and industry engineers having intensive discussions about the realities of present and future technology.

"I have been pleasantly surprised," said Chrysler Corp. research and materials engineering director Charles M. Heinen. "For 10 years, I have attended government-industry meetings, and for the first time there appears to be a possibility that the government is abandoning its posture that any research information coming from industry is suspect.

"This is the beginning of an era that can put us on the level of other countries where technical people from industry and government work together to solve what are essentially technical problems," Heinen added.

He pointed out that government regulators were the people at this conference who asked about the limits of technology "and who even suggested that maybe we should be looking at 40 or 45 mpg cars instead of 50 mpg by the end of this century."

He added that the ultimate decision over how fuel efficient and technologically advanced the car of 2000 will be is up to the public. He estimated that the cost to the nation of developing the 50-mpg fleet would be "around $100 billion."

"I can't tell you how pleased I am," Secretary Adams told the closing meeting of the conference Wednesday after being briefed by the leaders of the three discussion sessions.

"I am convinced we can achieve a substantial improvement in mileage that indicates that we really are going to be able to get the total breakthrough we will need for full mobility in the year 2000," Adams said.

He also said he will take back to President Carter a "research agenda" proposed by the gathering.

"I think we can get mileage in the range of 40 to 50 mpg, and it is realistic to try. It is incredible to me that a public-policy issue of this size has been left alone so long."

Adams said he will use the agenda proposed Wednesday as the groundwork for a follow-up meeting with auto industry leaders in Washington April 17.

The leaders of the three discussion groups outlined the conclusions of their panels at the closing session. Their findings:

Based on the information presented, the panel on engines was unable to identify a specific engine technology that it could call superior with respect to fuel efficiency, exhaust emissions and general acceptability for meeting a range for public needs. But it concluded that research emphasis currently given to the type of engines already in use, including diesels, has not been sufficient.

"It is clear," the panel said in its report, "that improvements in fuel efficiency can be achieved for the in-use category of engines, even for mature and widely used engine systems, and that these improvements can provide very large fuel conservation benefits to the nation."

The panel said further research into the diesel engine is needed, not to improve its "already good fuel economy, but rather to reduce its nitrogen oxides and particulate (soot) emissions."

The panel did call for some research into alternative concepts, including an external combustion engine, the gas turbine and others.

The materials and vehicle structures panel felt that in order for domestic fleets to average 50 miles per gallon their average weight will have to drop to between 2,000 and 2,200 pounds. Existing fleets average more than 3,000 pounds.

The panel called for increased use of lighter composite materials, lightweight alloys and aluminum, as well as structural improvements to reach the 2,000-pound goal.

Technology exists, the panel contended, to lower the weight of cars up to 40 percent through the use of lighter materials, redesigned components and the combination of functions, like the use of a turbocharger to both run an air conditioner and boost engine power.

The fuel and power train panel call for more basic research into the combustion process and how it works. A better understanding of that process will help those trying to improve it, the panel noted.

The panel also called for research into alternative power plants, including solar and electric engines where primary research should be devoted to increasing power storage capacity of batteries.

Panel members will be conferring over the next several weeks to iron out their proposals for Adams to use in working with industry.