Americans spent an average of $2,631 to own and operate their automobiles last year, $279 or 12 percent more than in 1979, according to a Hertz Corp. study released yesterday.

One-third of the increase in vehicle operating expense in 1980 was due to rising fuel costs, the Hertz study reported. Faced with the increase in gasoline and diesel fuel costs, car and truck owners cut back even further on driving last year. Passenger car travel fell nearly 6 percent last year, based on mileage estimates by Hertz, including a 12 percent drop in vacations and other personal driving.

The cost of car ownership in the past decade has risen considerably faster than the overall inflation rate, Hertz said. "Since 1972, the last full year before the Arab oil embargo, average per-unit truck costs are up 166 percent and passenger car expenses have risen nearly 110 percent," in spite of the reduction in the amount of driving since then, Hertz said. "During the same period, the nation's overall inflation rate was 89 percent," it added.

The other primary factors in the increase in vehicle operating costs last year were: interest charges, up 18 percent; maintenance and parts, up 15 percent; insurance and licensing fees, up 14 percent, and depreciation, up 9.4 percent.

Among company executives and industry analysts, there is a continuing, unresolved debate over the impact of higher car prices and operating costs on the demand for new cars. Industry officials say they remain confident that the long slump in new car sales will end once interest rates come down significantly -- and stay down. A few analysts, however, say that the combination of rising prices and costs has caused a broad change in consumers' attitudes, leading them to hold on to their present cars for longer periods.

General Motors Corp., the industry leader, served notice last week that it doesn't believe it has run into significant price resistance, announcing that prices on its 1982 models will increase an average of $507, or 4.8 percent over current prices.

That will bring the sticker price of an average GM car to over $10,000.

"This increase is substantially less than GM's cost increases for material and labor, which have exceeded 10 percent annually," GM said.

Part of the sticker price increase is due to an ungrading of the standard equipment provided on all new models, GM said. On the 1982 Chevrolet Citation, electronic fuel injection and high-pressure tires have become standard equipment rather than extra-cost options.