The heavy-metal days are winding down in the U.S. auto industry.

American car companies, which still are under federal and consumer pressure to turn out lighter, more efficient vehicles, are making increasing use of plastics and other lightweight materials to do the job.

Cast iron, steel and other heavy metals will continue to constitute most of the weight in the "average" U.S.-made car by 1995, but they will be far less of a factor in a car's weight than they are today, according to auto industry analysts at the big accounting firm of Arthur Andersen & Co.

The average U.S. car, which now weighs 3,202 pounds, contains about 1,500 pounds of basic steel, according to the Andersen report. A comparable car will weigh about 2,917 pounds in 1995 and will have 1,225 pounds of basic steel, about 18 percent less than in today's models, Andersen said in a recent report on competition in the U.S. car market.

Cast-iron content in domestic automobiles will fall 20 percent over the next 10 years, the report said.

By comparison, the use of plastics in passenger cars will increase 28 percent, from 215 pounds per car this year to 275 pounds per car in 1995. The use of cast-aluminum parts will increase 13 percent over the period. There will be a 12 percent increase in the use of high-strength, lightweight steel; and there will be a total 25 percent decrease in glass and other weighty, nonsteel materials, the Andersen analysts predict.

The decline of heavy metals in U.S. auto manufacturing will aggravate already poor sales in the nation's steel industry, which supplied 12.6 million tons of steel to domestic car makers in 1984.

In the 1990s, American car companies will use between 6 million and 9 million tons of domestically produced steel in their cars and trucks, said John C. Tumazos, a steel industry analyst for New York-based Oppenheimer & Co. Inc.

But the bad news for steel will be good for new-car buyers, auto industry analysts and officials said in recent interviews.

Cars that weigh less get more miles per gallon. Cars with bolted-on plastic shells, such as the Pontiac Fiero, generally are easier to repair than their metal-covered counterparts. Damaged plastic skins can be repaired or replaced more quickly than metal.

But the use of lightweight materials will not stop with the car's body. U.S. and Japanese auto makers are experimenting with ceramic engines and components. The idea is to come up with a light engine strong enough to withstand intense heat and pressure while pumping out as much, or more, horsepower than the heavier metal engines produce.

Ceramics are lightweight materials. They also are very hard, so they resist wear. They withstand heat better than metal, and this is a desirable property in an internal combustion engine, because higher operating temperatures improve efficiency. Conceivably, use of ceramics could lead to an engine that could operate without a cooling system.

Moreover, it may be possible to turn the extra heat into useful energy, generating still more power and fuel efficiency.

The problem for the U.S. auto makers is that the Japanese are beating them in the ceramics-engine department.

Nissan Motor Co. Ltd. is making 500 ceramic turbochargers per month for its 300 ZX sports cars sold in Japan. Turbochargers force more air into small engines, thereby boosting engine compression and combustion efficiency. Because turbochargers generate so much additional engine heat, a ceramics application on those components is ideal.

Isuzu Motors Ltd. plans to introduce a 1.8-liter ceramic diesel engine by 1990. That engine is expected to be 30 percent more fuel efficient than current comparable automotive powerplants, according to U.S. and Japanese automotive engineers.

The Japanese move into automotive ceramics is producing some consternation among auto makers and suppliers in this country. It also is creating a golden opportunity for the little-known domestic ceramics industry. "Ceramics is a key technology which will determine whether a U.S. buyer will buy a foreign car or a domestic car," said Carr Lane Quackenbush, technical director at Norton-TRW Ceramics in Northboro, Mass. The newly created company is a joint venture between Norton Co. of Worcester, Mass., and TRW Inc.

Lighter-weight, more-fuel-efficient, high-performance cars presumably will have high consumer appeal, Quackenbush said. The cars also could have lower initial costs, because they would be manufactured without traditional components now needed to control engine heat, he said.

The Japanese are moving ahead of their American rivals in automotive ceramics, largely because Japan's government and auto makers "see this technology as the wave of the future, and are willing to take short-term losses" to develop a product that will bring long-term gains, Quackenbush said.

U.S. ceramics manufacturers this year formed the U.S. Advanced Ceramics Association to lobby the government for grants and legislation to help develop a domestic automotive ceramics industry.

Domestic car companies, meanwhile, seem to be concentrating on coming up with newer and better plastics to take the place of metals.

"General Motors Corp. appears increasingly committed to substituting plastics for steel in automobiles," said Oppenheimer analyst Tumazos. The reasons are that "GM seeks to engineer high-cost labor out of its American production."

GM also "escapes high-cost domestic steel and steel labor rates when it uses plastics," Tumazos said in Oppenheimer's latest published review of the domestic steel industry.

Plastic body panels reduce labor costs because their manufacturing process -- injection molding and subsequent milling -- replaces more costly conventional stamping presses and assembly lines, Tumazos said. Auto makers can save nearly $250 per car at current compensation rates by dropping metal bodies in favor of plastic skins, Tumazos said.

But domestic auto makers argue that it is impractical to use plastic shells on anything other than limited-production models: It takes three to four minutes to produce a plastic body panel compared with one to two seconds to stamp out a comparable metal part. Domestic auto industry officials say that the production time for plastic bodies needs to be cut to at least one minute to be useful in large-volume car assembly.

"We're challenging the plastics industry" to come up with technology to speed up the process, said Mary Elliott, a spokeswoman for GM's Chevrolet, Pontiac GM of Canada (CPC) Group. "There's no doubt that the use of plastics will increase in body panels in the future. But we're not there yet for the large volumes."

However, Tumazos suggested that GM is bluffing to conceal its product plans. "We believe that GM has figured out how to overcome this hurdle" of the relatively lengthy production time for plastic panels, he said.

Tumazos said suppliers' reports indicate that GM soon plans to begin installing plastic shells on its Chevrolet Camaro sports cars, which he said will be produced at the rate of 250,000 a year.

GM officials declined comment.