After the cost of buying a car comes the cost of maintaining it

If you ever really figured out what it costs you each year to own and operate your car, you'd probably flag down a cab. The American Automobile Association (AAA) figures the cost at 23 cents a mile, but other groups arrive at different figures. Take your pick.

If you drive a full-size car, such as a 6-cylinder, 4-door Chevrolet Impala, for instance, the figure is 34.2 cents a mile if you drive 10,000 miles a year, 25.7 cents if you drive 15,000 miles a year, and 23.2 cents at 20,000 miles a year, according to the AAA.

Going down the scale in size, a midsize car (in this case a 6-cylinder, 4 -door Chevrolet Celebrity sedan) checks out at 31.2 cents for 10,000 miles, ranging down to 21.2 cents for 20,000 miles. A Chevrolet Chevette subcompact is 27 cents and 18 cents for 10,000 miles and 20,000 miles, respectively.

All are 1984-model vehicles, and the 23-cent-a-mile figure is a composite national average for the three cars, according to the AAA.

Obviously the way you drive, the condition of the car and transmission, the level of power equipment on the car, the need for air conditioning, the roads you use, the weather, tires, and a host of other variables enter into the mix.

It probably costs far more to drive a car than most motorists ever realize. After shelter and food, it's the biggest item in the average family's budget.

The highest total cost per mile driven was found in the six New England states, mainly in the fixed costs of insurance, depreciation, taxes, and financing. The lowest costs were in the Southeast, running from Florida to Louisiana, up to Kentucky and Tennessee, and east to the Carolinas.

Despite the cost, auto travel is on the rise, the AAA reports.

The Hertz Corporation reports that American motorists spent $341.8 billion last year to own and operate their cars, down from $350.7 billion in 1982, the first peacetime drop in history. (The report on 1984 costs won't be out till next summer.)

Hertz put the the average per-mile cost in 1983 at 33 cents a mile for the estimated 8,317 miles that the average car traveled in 12 months, down 6.8 percent from 35.42 cents the year before.

The average age of cars on the American road rose again in 1983 for the 14th consecutive year - to 7.4 years from 7.2 years in 1982, says Hertz. As a result, the cost of depreciation continues to fall, because a car loses up to half its value in its first three years.

Helping the motorist reduce his variable costs - fuel, oil, upkeep, and tires - are the higher-mileage cars now coming off the assembly lines as well as the stable or falling cost of gasoline.

Do you keep an eye on the kind of fuel mileage your car delivers on the road? If not, check it out sometime. Fill the fuel tank to the brim, make a note of the mileage on the odometer or reset the trip gauge to zero, and drive till it's time to refill the tank. Then figure out how many miles the car has gone and divide into the elapsed mileage from fill-up to fill-up. That's the mileage you're getting at that time.

It will vary as you encounter different driving conditions throughout the year. Perhaps you have to rely rather heavily on the air conditioner when the weather is hot. The AC unit will reduce the mileage by up to 10 percent.

As the cost of buying a new car goes up, it makes sense to cut back on the cost of operating it on the road. Prudent driving habits will make a big difference. Keeping up ''with the other guy on the road,'' particularly if it means exceeding the national speed limit of 55 m.p.h., will cost you significantly more in fuel costs and frustration than staying within the letter of the law.

A car at 70 m.p.h will get up to 20 percent less distance on a gallon of fuel than a car running at 55.

To keep a car in tiptop running condition also reduces the driving cost. The car's manual explains the recommended time and distance between car-maintenance procedures.

You might want to draw up a work-sheet to pinpoint what it actually costs you to drive your car, or several cars, if you have them. Include not only the original total cost of the car, but also the depreciation, insurance, vehicle registration fees and driver's license costs, interest on a car loan, maintenance, fuel and oil, tires, tolls, garaging, and anything else that might add to the cost of running a car.

Every car loses its value at a different rate, depending on how it was driven , the distance, and the demand for it in the marketplace.

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