The automobile industry's 1985-model clearance sales with cut-rate financing helped boost retail sales a strong 2.7 percent last month and were a major factor in lowering producer prices for finished goods by 0.6 percent, the government reported yesterday.

The drop in finished-goods prices left the index up only 0.2 percent from September 1984, and down 0.1 percent since the beginning of this year. Last month's decline was the largest since a 0.7 percent drop in January 1983.

The index has not gone down for a full year since 1963, the Labor Department said.

Passenger car prices fell 3.8 percent on a seasonally adjusted basis and were down 1.3 percent from their year-earlier levels. Consumer food prices went down 0.9 percent, the second monthly decline in a row, and were 2.4 percent lower than in September 1984.

The Commerce Department reported that the 2.7 percent increase in retail sales, to $120.2 billion, compared with a 2.3 percent rise in August, when the auto sales began. Sales of autos were up 9.6 percent to $28.8 billion.

Sales of goods other than at automotive dealers increased 0.6 percent in September, down from a 1 percent gain the month before.

The overall increase in sales was the largest since a 3.1 percent gain in April, Commerce said.

Most analysts believe the rise in consumer spending will slow, even though some auto manufacturers already have instituted cut-rate financing on new 1986 models. Personal income has not been growing rapidly and consumer debt has increased substantially faster than income in the past year.

Lea Tyler, an economist with Chase Econometrics, said the September high level of car sales is likely to fall as car makers eliminate their inventories of 1985 models.

The data on some other sales were not so bullish either, particularly at department stores, where sales fell 2 percent. "The department stores sales are disappointing," Tyler said. "I would have expected them to be flat, but not to have gone down."

Most of a 0.5 percent decline in finished-consumer-goods prices was due to a 3.8 percent drop in passenger car prices and a 2 percent drop in light-truck prices, the Labor Department said. The index for finished capital goods fell 0.6 percent, largely a result of lower prices on cars and trucks bought by businesses.

Finished-energy-goods prices fell 0.1 percent, with lower gasoline prices more than offsetting an increase in the cost of home heating oil. Even after last month's changes, gasoline prices in September were 2.3 percent higher than a year earlier while heating oil prices were 2.4 percent lower.

The index for intermediate goods showed no change in September, as a lower foods and feeds prices were matched by increases elsewhere.