Japanese automakers that have done the best job of producing fuel-efficient cars and trucks stand to suffer most under legislation before the U.S. Senate that would boost the mileage standards on new vehicles.

That, at least, is the Japanese automakers' explanation for their unusual show of unity with American car companies yesterday in condemning a bill that would force car manufacturers to boost their fuel efficiency by 40 percent by the year 2001.

Under current federal fuel-economy standards, the fleet of new cars sold by each automaker is required to average 27.5 miles per gallon. That puts companies like General Motors Corp. and Ford Motor Co. at a disadvantage, because they sell many big cars that pull down their fuel-efficiency average.

Conversely, the current system benefits the Japanese, since they produce mostly small cars that get better mileage.

But legislation being proposed by Sen. Richard H. Bryan (D-Nev.) would require that each automaker improve on its own performance -- by 20 percent by 1995 and another 20 percent by the year 2001.

As a result, the Bryan proposal would in effect set different standards for different companies, penalizing those that have already done the best job at getting more miles per gallon from their cars, according to critics of the bill.

"It's like having an 'A' student and a 'C' student in the same class and telling them both that they have to improve their performance by identical levels. How is that fair to the 'A' student? What is the 'A' student supposed to do?" asked Toni Harrington, Washington representative of Honda Motor Co.

Honda cars now sold in the United States average 30.8 miles per gallon, 3.3 mpg better than the current federal fuel-economy standard. Bryan's bill means that Honda will have to achieve nearly 45 mpg by 2001, compared with 40 mpg for Ford and GM, which now average 26.4 mpg and 27.1 mpg respectively, according to Harrington and other industry officials.

"We have to improve a greater amount, because we're starting from a higher base," said Harrington, who added that her company joined GM in protesting the measure to avoid a split within the industry on the bill.

George Nield, president of the Association of International Automobile Manufacturers, which represents 20 foreign car companies, most of them Japanese, agreed.

"No one in the automotive industry believes that the standards proposed in the Bryan bill are achievable without dramatically reducing the size and weight of vehicles," Nield said. The problem is that many Japanese vehicles are already so small, Japanese automakers are reluctant to cut their sizes further, Nield said.

James Mulhall, spokesman for Bryan, dismissed criticism that the bill unfairly favors domestic manufacturers over Japanese and other foreign competitors.

"That's simply not the case," he said. "We asked for a 40 percent, across-the-board increase regardless of where a firm is now. That's fundamentally fair," because, on the whole, "there is no massive gap" between foreign and domestic cars on fuel efficiency, Mulhall said.

The "flip side of that fairness question" is that averaging, the current method of establishing fuel-economy standards, penalizes companies that provide a broad range of vehicles, Mulhall said.

William McGavern, a lawyer with the U.S. Public Interest Research Group, a consumer organization supporting the proposal, conceded that companies like Honda are doing a good job. But he said that they can do better and that domestic automakers can make the greatest improvements.

"We certainly hope that this bill will give the American companies the kick in the pants that they need to innovate," McGavern said.