Only 14 percent of all auto drivers in the U.S. use their safety belts, according to a new study by the Transportation Department. But in cars equipped with automatic belts, like the Volkswagen Rabbit, 78 percent used their belts.

In a second study, the Oldsmobile Cutlass and the American Motors Pacer were shown to have the least comfortable and convenient seat belts on the road of 30 cars tested.

Joan Claybrook, administrator of the National Highway Traffic Safety Administration, said, "We are very concerned that all possible steps be taken to increase belt usage . . . the most important safety measure that individual motorist can take is to fasten their safety belts."

The belt usage survey, conducted for NHTSA by Opinion Research Corp., of Princeton, NJ., showed that in 1974-78, cars equipped with lap and shoulder belt combination systems, use was 14 percent (12.6 percent for bothe belts and 1.4 percent for the lap portion alone.)

In earlier models (1968-1973) equipped with separate lap and shoulder belts, usage of both was only 3.3 percent, with lap belts only being used by 13.7 percent.

In cars equipped with passive-or automatic-belt systems, like the Volkswagen Rabbit, usage was up to 78 percent.

The study also showed that for 1976-1978 model cars:

Motorists driving in subcompacts buckled up in 19.5 percent of the cars tested, while that number dropped to 12.5 percent for compact car drivers, 10.3 percent for intermediate car drivers and 9.6 percent for drivers of full-sized autos.

Substantially more drivers in the West (18.3 percent) use belts than in other parts of the country (11.4 percent).

Significantly more women drivers (16.4 percent) use belts than men (12.6 percent).

Cars equipped with continuous light-buzzers and starter interlocks (1972-74 models) has their drivers buckled up 17 percent of the time.

Usage in 1976-78 cars-which have eight-second light and buzzer systems-was about 13.5 percent, "indicating," the study said, "that the [buzzer systems] has little effect on belt usage."

The second study evaluated 30 cars and their seatbelt systems for comfort and convenience over a 4-day period last September, and was conducted by Verve Research Corp., of Rockville.

The study dealt with such problems as twisted belts, excessive slack and incomplete retraction.

The best score went to a Ford Fairmont and models of the Subaru and Mazda.

Other cars evaluated and ranked in order after the top three are: DeVille, Mercedes, Corolla, Datsun, Volvo, '78 Impala, Granada, '75 Impala, Volare, Civic, Fiat, LTD, Rabbit, St. Regis, Mustang, Pinto, Chevette, Cordoba, Omni, LTD II, Camaro, BMW 320, Concord, Cutlass, Pacer.