A federal research group has recommended a more liberal policy on truck weights that it said could save $4.3 billion a year in transport expenses, even though it could cost railroads $750 million in lost revenue.

The National Research Council committee said all states should be given the option of exempting some trucks from a federal limit of 80,000 pounds on the nation's interstate highways. It also recommended changes in the rules for bridge use that would allow heavier concrete mixers, dump trucks and other trucks with shorter wheelbases.

The weight exemptions would be given on condition that they include fees to cover pavement and bridge wear, said the committee, made up of transportation experts from universities, the industry and government.

The research council is linked to the National Academy of Sciences and the National Academy of Engineering.

The committee report recommends that heavy trucks still be restricted to designated routes, comply with all other federal weight restrictions and comply with strict safety standards.

The exemptions would still be up to individual states, but the report said it was not fair to prevent about half the states from allowing exemptions while allowing others who had exemptions before the current law to keep theirs.

If all states adopted the exemptions, annual transportation savings for truckers could total $1.9 billion because of the use of more productive vehicles, the report said. This would lower costs by 1.6 percent, attracting $750 million in business that now goes to railroads, it said.

S vings in trucking costs from a new bridge formula could total $2.4 billion, the committee said.

The report, requested by Congress, is expected to play a key role in shaping the Capitol Hill debate on truck weight limits. The study brought praise from the trucking industry, which estimates its services at $240 billion annually.

A rail industry spokesman said he had not seen the report but was concerned that any nationwide increase in truck sizes and weights could have a serious impact on railroads.

Dan Lang, vice president of the Association of American Railroads, said trucks pay only about half their share of highway maintenance costs and fail to pay other highway expenses, including the social cost of pollution and added traffic.

"If you're going to go out and upset the status quo this way, you should really take another look at all the costs associated," Lang said.

The committee report did not call for substantially heavier trucks over 80,000 pounds and did not address truck lengths, said its chairman, Lester A. Hoel of the University of Virginia.

"We simply recommended a level playing field for fairness, to give all states the option of implementing a permit program," Hoel said.