Just what kind of cars will be built in the next decade may depend on which we decide is more important: safety or fuel economy.

Do we want the safest possible car on the road, or the car that uses the least gasoline? We probably can't have both.

Most auto engineers agree that for the 1980s, Americans will still be using gasoline-powered autos. Electrics are years away from being practical. Diesels, while increasingly popular, still have emmissions problems. And alternative fuels like gasohol rely on gasoline engine technology.

But the bottom line for the '80s will be performance. Downsizing and weight reduction are at the top of the automakers' lists to improve mileage. Existing law mandates that U.S. automakers build fleets averaging 27.5 miles per gallon by 1985.

The car of the '80s will still have nearly all its structural parts made of steel, although there will be more use of aluminum and plastics. Most of the cars will have front-wheel drive.

The trend toward safer cars could be severely undercut by the argument for better mileage. Lighter cars are generally more dangerous, and many safety features sought by federal regulators will add considerable weight to cars.

Last February, more than 700 auto industry engineers, scientists and government regulators met in Boston.

The meeting grew out of a Dec. 5 speech Transportation Secretary Brock Adams made to the Economic Club of Detroit, in which he asked the auto industry to work with government to "reinvent the car."

The panel discussing future engines at the meeting was unable to come up with a specific technology superior in all areas: fuel efficiency, exhaust emissions and acceptability for meeting a wide range of public needs. For the '80s, the panel said, "Improvements in fuel efficiency can be achieved for the (existing) engines, (and they will) provide very large fuel conservation benefits to the nation."

The materials and structures panel called for increased use of lighter composite materials, such as alloys and aluminum. Technology exists, the panel reported, to cut auto weight 40 percent.

The fuel and power-train group called for more research about the combustion process, conceeding the internal combustion engine is going to be around a while.

Clearly the tone of the Boston meeting, and the subsequent tone of government research efforts, shows there will be few fundamental changes in the cars of the '80s, only a steady refinment of existing systems.