Relax, Rolls-Royce lovers: Uncle Sam had decided not to outlaw your Silver Shadow, even if it is an unrepentant gas guzzler.

The 140-mile-an-hour, 12-mile-a-gallon Maserati and the anything-but-economical Excalliber are also off the endagered automobiles list, thanks to the U.S. Court of Appeals.

But driving an exotic energy-waster is going to cost you.

There's a $550 federal gas-guzzler tax included in the sticker price of a 1980 Rolls or Maserati, and the penalty for conspicuous consumption of gasoline will be going up faster than the price of fuel.

By 1985, the federal government will be collecting a tax of $3,850 on every car sold that gets less than 12.5 miles to the gallon.

Until the Court of Appeals ruled earlier this month, however, there was a chance that some low-MPG cars not only would be taxed, but outlawed altogether.

Upholding a luxury-car loophole in the law that sets fule economy standards for autos, the court ruled it was all right to exempt cars manufactured in small numbers.

The appeals panel threw out a lawsuit challenging the small-manufacturer exemption filed against the National Highway Traffic Safety Administration by Ralph Nader's Center for Auto Safety.

The decision was unanimous, but unpopular even with the julges who made it.

"By exempting these foreign manufacturers of luxury cars which burn inordinate amounts of gasoline (Rolls Royce, Excallibur and Maserati) from its gasoline conversation regulations, NGTSA makes a mockery of our professed attempts to reduce gasoline consumption," said Chief Judge J. Skelly Wright.

"These exemptions not only promote wste of gasoline; they increasse our air pollution as well as our foreign trade deficit," he added, suggesting "Congress may want to reconsider this matter."

The court noted that "however unwise one may find exemptions of luxury cars, they are not precluded" by the federal law.

After the Arab oil embargo in 1974, Congress passed a law requiring car makers to improve their corporate s average fuel economy (CAFE) every year. Vigorously opposed by Detroit, the law forced auto companies to launch a multi-billion-dollar downsizing program to design and build smaller, more efficient cars.

Because of the high cost of designing new engines -- or whole new cars -- to get better mileage, Congress wrote in an exemption for manufacturers who produce less than 10,000 vehicles a year.

Producers of the high cost of designing new engines -- or whole new cars were the first to seek exemptions -- Rolls Royce, Maserati, Excalibur and Aston Martin. Also unable to meet the mileage barriers were two small American producers -- Checker, the taxi-cab company, and Avanti, which still makes the former Studebaker model in small numbers.

Although they make some of the most expensive cars in the world the small companies make so few of them that they can't afford to retool to comply with the fuel company law, the NHTSA has ruled.

All told, the exempt makers sold less than 10,000 cars in the United States last year, said NHTSA counsel Steven Wood. "The amount of gasoline involved (in the exemption) is just miniscule," he added.

While the small makers are exempt from the CAFE rules, NHTSA officials point out that they aren't exempt from the other measure meant to cut consumption of fuel -- the gas guzzler tax.

Any 1980 model that gets less than 15 miles per gallon is subject to the tax; the more gas it burns, the bigger the tax.

None of the major American car makers are paying the tax this year, and none of them are expected to because even Detroit's biggest cars get better mileagee than the law demands, federal officials say.

For 1980 models the gas guzzler tax is $200 on cars that get between 14 and 15 mpg, $300 for 13 to 14 mpg. and $550 for less than 13 mpg. Excadiburs carry a $200 penalty this year, and the full $550 is being paid on cars made by Maserati, Aston Martin of England, and Rolls Royce, whose models average less than 11 mpg.

The gas-guzzler tax rate and the mileage need to escape it increase every year until 1986.

By then any car that gets less than 22.5 miles to the gallon will carry some special tax, and cars that get less than 12.5 mpg will be penalized by $3,850.

"We felt that was enough," said NHTA's Wood, although federal officials admit the tax isn't likely to discourage any Rolls Royce buyer.

For those who haven't kicked tires at their friendly Rolls Royce dealer lately, the list price of a Silver Shadow II -- not the most expensive model -- has more than doubled in the last three years.

You could pick up a new '76 Shadow for $38,750, Brooks Honeycutt, president of Brown Rolls Royce of Vienna, told the people on his mailing list recently.

The 1979 sticker price was up to $78,357 and "the car is projected to sell for approximately $93,000 by the fall of 1980," Honeycutt told potential buyers, "If you would like additional information on the Rolls-Royce motor car line, just have your secretary contact us."

He didn't even mention the gas-guzzler tax. CAPTION: Picture, Your basic $77,600 model Rolls Royce at Brown Lincoln-Mercury in Arlington. By Fred Sweets -- The Washington Post