Amid stiffening congressional opposition to tougher fuel economy legislation, the EPA Tuesday named the Geo Metro XFI - a Japanese-made General Motors import - the most miserly of the 1991 model-year cars at 50-plus miles per gallon.

Japanese-built cars once again dominated the annual gasoline mileage ratings, winning nine of the top 10 spots. The German-made Volkswagen Jetta was the only non-Japanese car among the top 10.The mileage guide was released by the Environmental Protection Agency as legislation to increase federal fuel economy standards bogged down in the Senate. Backers of the bill failed to get the 60 votes needed to limit debate, handing opponents a victory in their efforts to prevent action on the bill. That could kill the bill for this congressional session, which is nearing its end.

The bill, strongly opposed by automakers and the Bush administration, would raise the fuel efficiency standards for new cars to a fleetwide average of 40.1 mpg by 2001, up from the current 27.5 mpg.

The EPA said the 1991 model-year fleet is expected to average 28.1 mpg, up from the 27.8 mpg estimate for 1990 cars. However, the slight increase for 1991 cars still leaves the fleetwide average short of the 1988 industry-wide average of 28.6 mpg.

The decline in fuel economy over the last two years has been cited by energy conservation proponents as clear evidence the government must force Detroit to make sigbificant fuel economy improvements above the current 27.5 mpg federal standard.

For individual carmakers, the Geo line, manufactured by Suzuki and imported and sold by GM, made the best showing of any brand name, taking four of the top 10 spots on the EPA's mileage list. Honda and Suzuki took two spots apiece on the top 10 list.

The EPA said the top-rated Geo Metro XFI gets 53 mpg in city driving and 58 mpg on the highway. It was the fifth year in a row that the best gasoline mileage was posted by the Geo Metro XFI, a tiny 1,875-pound, three-cylinder subcompact.

The biggest gas guzzler among 1991 models was the Lamborghini DB132-Diablo, a high-powered sports car that logged 9 mpg in the city, 14 mpg highway.

In ranking cars by class, the EPA said:

-For "midsize" cars, Chrysler's Acclaim and Spirit were the most fuel-efficient, both averaging 24 mpg in the city and 34 on the highway. Three Rolls Royce models were the worst, all averaging 10 mpg city and 13 highway.

-For "large" cars, the best was the Saab 9000 at 20 mpg city and 26 highway, and the worst was Cadillac Brougham at 16 mpg city and 25 highway.

-For "large station wagons," the best was Ford's Grand Marquis and LTD Crown, both 17 mpg city and 24 highway, and the worst was GM's Caprice Wagon, Custom Cruiser and Roadmaster models, all getting 16 mpg city and 25 highway.

EPA officials noted the agency's fuel mileage ratings were based on laboratory tests under optimum conditions and on-the-road results could vary.

In the showdown on Capitol Hill over fuel economy legislation, senators voted 57-42 in favor of limiting debate on the bill - three short of the needed 60.

That provided a big boost for the filibuster effort led by Sen. Don Riegle, D-Mich., whose home-state automakers strongly oppose the bill. The vote could enable Riegle to block Senate action on the bill this year, forcing proponents to start over next year.

Even if measure gets through the Senate, its outlook is bleak in the House, where it likely will be sent to the House Energy and Commerce Committee. That committee is headed by Rep. John Dingell, another Michigan Democrat protective of Detroit.

Automakers and the administration oppose the bill as unrealistic, saying the technology does not exist to make big fuel economy improvements. They maintain the only way to meet the bill's requirements would be to make cars smaller and lgihter, which could lessen highway safety, restrict customer choice at the car lot, hurt sales and cost auto industry jobs.

The bill has gained momentum since the Persian Gulf crisis, which has underlined America's heavy dependence on foreign oil and re-energized government oil conservation efforts.

Backers of the bill say that with motor vehicles accounting for 40 percent of U.S. oil consumption, the nation will never reduce its reliance on imported oil unless cars are more fuel-efficient.