Interest rates on new-car loans have been edging down a bit at banks and S&Ls, and should drop even more. One reason: cutthroat competition from auto dealers, who are now offering bare-bones loans on their smaller American cars.

Dealers for Ford, Chrysler and General Motors can offer as little as 8.8 percent financing on certain compacts or subcompacts bought right away. At present, the GM and Chrysler deals are slated to expire on April 30, and the Ford deal on April 22.

American Motors has cut rates on some loans to 8.5 percent (good through the end of April), and is offering a better warranty besides.

If you bought a Chrysler car or truck anytime between 1979 and l984, the company will give you a $500 "gift certificate" to cut the cost of a new U.S.-made Chrysler, Dodge or Plymouth bought by June 6.

The last time that car dealers went in for serious discounting, the bankers were forced to drop their rates too, so as not to lose business. If the auto-dealer discounts continue, the same thing should happen again.

But there's more than one way to skin a cat, or a consumer. Traditionally, car loans have no add-on fees. But now, banks all over the country are suddenly inventing "processing fees," "filing fees," "origination fees" and other fanciful charges to raise your cost through the back door.

These fees range from around $15 to $50. If they're charged only to people who actually borrow money (as opposed to everyone who simply applies for a loan), their cost should be counted when figuring the annual percentage interest rate that you're quoted on the loan. But Consumer Action, based in San Francisco, says that some banks were neglecting to do so -- which made their loans look cheaper than they really were.

A majority of banks and S&Ls still have no add-on fees, so you can cut costs by sticking with them. And there are some other ways to save:

* Although some banks and S&Ls soak you on new-car loans, others actively seek your business with attractive rates. An 11-city survey by the Bank Rate Monitor, conducted in late March, showed huge differences even within the same city. For example, the Amalgamated Bank in New York charges a 12 percent fixed rate, while Citibank charges 16.5 percent on the same loan. Similar spreads can be found in other cities, meaning big savings for car buyers who shop around.

* You may be able to do even better -- for initial payments, at least -- by taking a variable-rate loan. Two banks in the survey, Michigan National in Detroit and Horizon Financial in Southampton, Pa., charge walk-in borrowers (who are not already customers) 11 percent -- the lowest in the survey. The lowest fixed rate was 12 percent, while the highest variable rate in the survey was 14.75, at Lloyds Bank in Los Angeles.

With a variable loan, your monthly payment generally doesn't change as interest rates go up or down. Instead, the term of the loan is adjusted. You'll pay a month or so longer if interest rates rise, and a month or so less if they decline.

* Some banks charge customers less than noncustomers. At Chicago's First National, customers pay 10.5 percent variable while people who simply walk in off the street pay 11.5 percent.

* It's common for lenders to charge lower rates on shorter-term loans. Some banks offer you only a small break on rates -- maybe a quarter of a point. But others give you larger savings. At Horizon Financial, a one-year new-car loan costs 12 percent, rising gradually to 13.5 percent for a five-year loan.

* The typical down payment for a new car is 20 percent. But a few banks will finance the entire purchase, at higher rates. Home Savings in Houston, for example, will lend the entire price of the car at 15 percent interest (for four years). If you put 20 percent down, you pay only 14 percent.

* A few imaginative lenders are attracting the affluent with special deals. Landmark Bank in Fort Lauderdale, Fla., lends money for luxury cars on a balloon loan. The borrower pays interest and a small amount of principal during the term of the loan, then pays the big balance all at once when the loan comes due. The money for the payment might come from selling the car, refinancing or trading it in and financing a new auto.

In form, the transaction is much like leasing, except that lease payments are not tax-deductible when you buy a car for personal (rather than business) use. Interest payments on a balloon loan, on the other hand, can be written off quite legitimately.