North American-Built Auto Sales Drop : Manufacturing: The Big Three report double-digit plunge. Honda leads Japanese firms to a 4.8% increase.

Sales of North American-built vehicles plunged 15.8% in mid-September, with double-digit drops by the Big Three auto makers more than offsetting a small rise in sales for Japanese companies, figures indicated Tuesday.

Sales of North American-made Japanese vehicles rose 4.8% in mid-September, fueled by a 25.6% boost in sales by Honda Motor Co.

But General Motors Corp., the world’s largest auto maker, reported a 20.9% drop in vehicle sales for the Sept. 11-20 period, compared to last year. Ford Motor Co. said its vehicle sales fell 17.4%.

Chrysler Corp.’s mid-September sales dropped an estimated 11.5%. The nation’s No. 3 auto maker only reports sales figures for the month. The Associated Press estimate is based on the auto maker’s average monthly market share during the previous 12 months.

The reports provided more depressing news for an industry mired in recession for more than a year. Dealers have been hoping for an increase in business, particularly as they clear lots of 1991 models and prepare for the 1992 season.

The new model year technically begins Oct. 1, but many auto makers have been shipping some new cars and trucks to dealers for months.

Spotting trends in vehicle sales at this time of year is tough. Auto makers are trying to run down inventories from the ending model year, frequently causing “fire sales” at dealerships, while trying to create excitement for the cars and trucks on their way to showrooms.

At the same time, and because inventories are down, consumers may not find exactly the car or truck they want. So they may postpone purchases until dealers can offer a better selection.

“We had a very good July,” said sales manager Ken Leary of Grappone Ford-Suzuki in Concord, N.H. “August dipped a little, but September is coming back.”

Through Sept. 20 of this year, sales of North American-made cars and trucks were running 12.5% behind last year, with Big Three sales down 15.1% and sales of Japanese vehicles up 8.8%.

GM said its car sales during the mid-September period dropped 24.1%, while truck sales fell 14.6%. For the year, GM’s sales of North American-made cars and trucks were down 14%.

Ford’s mid-September car sales were down 22.6% while truck sales were off 10.5%. So far this year, Ford’s combined car and truck sales declined 17.4%.

Among the Japanese auto makers, Honda’s sales for the year were up a slim 2.1%. Toyota’s mid-September sales of its North American-made cars were down 24.5% as the auto maker prepared to launch its new Camry mid-size car. So far this year, Toyota’s sales were down 5.5%.

Nissan said its car and truck sales for the period were down 21.4% and up 3.4% for the year. Mazda’s mid-September sales were down 9.9% and up 26.1% for the year.