Think you need a new car? Do a little figuring first

First the battery went, then the brakes, and now you find your car needs a new generator. Six months ago it was the shock absorbers. When will it all end? You wonder, is now the time to "start fresh" by investing in a brand-new car?

A new automobile is a major investment, increasingly so as prices for the same models go up many times a year. Now is a good time to make cost comparisons:

* How much does it cost to own and operate the car you have now -- gasoline, taxes, car payments (including interest), and insurance, for a year? If it is paid for, and the insurance and tax rates are decreasing annually, you are probably making some headway financially, you realize. This is, until you start to need major repairs.

* How much more a year would you be paying for expenses on the new car of your choice in the same period of time, including sales and excise taxes, gasoline, higher insurance costs, and loan-plus-interest payments?

* Based on your car's past record and its reputation on the market, what would be the estimated cost of repairs to keep the car running for, say, the next five years? Then compare the likely repairs that would be necessary on a new car with a good reputation, over the same period. What would be the estimated resale value of each at the end of that time?

* How much has the price of the new model increased in the past year? According to one automobile dealer, for example, the yearly price increase on its models is in the $300-$400 range.

Depending on your individual figuring, you may be surprised at how much more it costs a year to keep a new car on the road. One Massachusetts motorists who was recently considering the purchase of a new car sat down and did some rough estimating. She figured on buying a $5,000 car under a three-year loan. (Fortunately, the trade-in value of her present car was $2,000, which meant that she would only have to take out a loan for the additional $3,000 to purchase the new automobile.)

She found that several banks required 20 percent down on the total principal, and discovered that within that three-year period, she would be paying out approximately $1,500 on interest, additional costs on the excise tax, sales tax, and extra insurance coverage, including the minimum amount of insurance required by the bank before it would finance the new car -- all extra costs above what she would be spending on her already-paid-for 1975 car, with the exception of a few dollars a year for excise tax. (Taxes vary from state to state.)

The motorist decided to keep her old car, which was in relatively good shape. She reasoned that $1,500 would pay for a lot of repair work, and she would be free of the monthly payment on the principal (another $84 a month), as well.

Still other consumers want, and can afford, a new car and are willing to commit themselves to its purchase, regardless of the extra cost. And for those who can afford to pay outright for a new automobile, the loss is not as great.

Some further points to consider are:

* New car warranties cover most if not all repairs that are necessary to get the kinks out during the first year or so. (After the warranty runs out, the customer is virtually on his or her own, but many new car warranties these days are more extensive than ever before.)

* The Chances of needing major repair work during the first couple of years are unlikely, but if you should get a "lemon," there's no guarantee that the dealer will make good on the car once the warranty runs out.

* Even new cars decrease in value each year, but the resale value of a new car is greater than an old car.

* Generally, the older a car is, the less expensive it is to own, unless an unusual amount of repair work is required.

* After a certain amount of time and wear and tear, a motorist can expect some repair work on the car, whether he or she has an old or new car, and all cars need tune-ups regularly.

Now the question arises: If I decide to keep my old car, how much repair work is necessary to get it into reasonably good, safe shape?

The basic checkpoints are the engine, transmission, chassis, exhaust system, the front and rear ends, and the body, which should be checked for rust deterioration.

Other factors to consider are:

* Has the car been tuned up regularly during its "lifetime"?

* What is its repair record so far?

* Is your car a make and model that is known for its particularly good (or bad) maintenance record over a period of several years?

* Has the car ever been in an accident or abused through rough treatment?

* Does it get good mileage?

* Has most of the mileage accumulated on open roads or in congested city traffic?

* Is your car a model that is currently in demand due to its good gas mileage or other special features?

* Was it rustproofed when it was new?

* Are you interested in buying a new car for looks, comfort, or other personal reasons, or are you interested merely in a reliable, if somewhat battered, relatively cheap means of getting around?

The "Book value" of a new car would be higher than your old car, but any repair work done on your current model will increase its resale value, especially if you sell the car privately and not through a used-car dealer. And , really, it's what that "old clunker" is worth to youm that counts.

Other alternatives are to check discarded cars in junk yards for parts that are still in good condition, and to check on whether your repair shop will give you credit on old parts if you turn them in when new parts are installed.

Should you, too, decide to keep your old car, having a friend help you repair it is one way to keep costs at a minimum.

When your car is once again in top shape, keep it that way by:

* Having it tuned up at regular intervals.

* Giving the oil, transmission fluid, battery, and other vital parts a biweekly checkup, or as needed.

* Washing and waxing it regularly by hand to protect the finish.

* Garaging it whenever possible for extra protection.