The interest-rate war that boosted U.S. auto sales to record levels over the past six weeks began winding down yesterday along with the 1985-model year.

But the fighting is expected to resume again, perhaps as early as November, as the nation's Big Three car companies battle over customers for their higher-priced 1986 products.

General Motors Corp. and Ford Motor Co. yesterday ended their 7.7 percent interest-rate promotions that began Aug. 15. Chrysler Corp., the smallest of the Big Three, will end its 7.5 percent interest-rate campaign on Saturday.

All three programs, implemented to clear manufacturer and dealer lots of leftover 1985 models, were successful. Even foreign-car dealers -- many of whom for the past two years have arbitrarily added premiums to their prices -- were forced to try to match the U.S.-car financing rates.

But banks and other financial institutions generally stayed out of the fray. For example, interest rates on many Washington metropolitan area bank auto loans were at least 3.5 percentage points higher than those offered by the car companies.

"We didn't change our posture. We did not try to meet them head on with any kind of a promotional blitz," Richard Jennison, marketing vice president at Citizens' Bank of Maryland, said about the Big Three rates.

Citizens' "does not have the dollars" necessary to compete with multinational companies like GM and Ford in a finance-rate war, Jennison said. Also, banks generally lack the flexibility to exercise model-by-model discretion in setting rates on car loans, he said. The lower rates offered by the car companies usually applied to selected models.

U.S. auto sales reached three consecutive record highs during the interest-rate war. The latest available 10-day tally, for Sept. 11-20, was 289,582 passenger cars -- 32.32 percent above the 218,847 cars sold in the same period last year.

New 10-day figures are due today; and those, too, will show robust, incentive-fed sales, several auto industry analysts predicted yesterday. But the analysts said that the 1985-model incentive programs probably pulled many buyers out of the market for 1986-model cars.

That means that domestic auto makers will have a tough selling job to do, said Harvey Heinbach, an analyst with New York-based Merrill Lynch, Pierce, Fenner & Smith Inc.

"The incentive programs turned out to be more successful than we expected," Heinbach said. Merrill Lynch earlier this year predicted that U.S. auto makers would sell about 8 million cars during the 1985 calendar year. "Now, we're looking at domestic sales of 8.2 million," Heinbach said.

But he said that some of that increase was achieved at risk of lower fourth-quarter profits and lower new-model sales for the Big Three companies. That risk will be heightened by consumer reaction to 1986 car prices that will be substantially higher than those of 1985 models, Heinbach said.

GM, for example, is raising its average price by 3 percent; Ford, by 2.9 percent; and Chrysler announced yesterday that it is boosting its 1986 average car price by 2.7 percent over comparably equipped 1985 models.

The companies have declined to attach dollar amounts to those increases. But Automotive News, a Detroit-based car industry journal, estimates that the increases mean that average list prices will be up to $400 higher on 1986-model cars.

Those higher prices might be on the sticker, "but the companies are going to have to come off of them, in terms of discounts, in order to sell," Heinbach said.

GM, Ford and Chrysler officials reached yesterday all said that their companies have no immediate plans to renew incentive programs. But officials at all three companies indicated that they are prepared to use incentives again.

We'll evaluate programs like that on a daily basis," a GM spokesman said. "We'll be watching the sales."