Firestone Tire & Rubber Co. said yesterday it will close six of its U.S. plants by the end of October, eliminating the jobs of 7,000 current employes and another 1,500 workers who were laid off previously, in a major cutback of the company's passenger tire manufacturing capacity.

The Akron-based company is the third tire manufacturer this year to announce plant closings, following Uniroyal Inc. and Armstrong Rubber co.

Most of the plants produced biasply tires, once commonly used on passenger cars but now being rapidly eclipsed by radial tires, which provide better gasoline mileage and longer wear because of better construction.

Sales of bias-ply tires, and passenger tires generally, have been hurt by the slump in car sales, curtailed driving by motorists and rising automobile imports.

The Firestone plants to be closed are tire manufacturing units in Dayton and Barberton, Ohio; Los Angeles and Salinas, Calif.; and Pottstown, Pa., as well as a synthetic latex plant in Akron.

Peter Bommarito, president of the United Rubber Workers, said yesterday he was shocked that Firestone would take "such drastic action" without first asking the union for help in keeping the plants open.

The closings will cut Firestone's daily tire production by 34 percent, the rubber workers union estimated.

Bommarito said the union will meet with Firestone to seek ways of keeping the plants open. The rubber workers' 1979 contract requires the tire manufacturers to give six months' advance notice to plant closings and to consider alternatives to closing.

Union representatives are holding negotiations now with Uniroyal in an effort to prevent the closing of its plants in Detroit and Chicopee Falls, Mass.

Firestone Chairman Richard A. Riley warned at the company's annual meeting a month ago that his company and the entire industry was in "a state of change" and the "immediate outlook for our industry is undoubtedly difficult."

On Feb. 21, Firestone reported a loss of $13.8 million for the three months ended Jan. 31. The company had earned $34.8 million in the same period the year before.

Richard Haydon, a tire industry analyst at Goldman Sachs & Co., said Firestone and other manufacturers were ending a period of contraction, eliminating bias-ply facilities that no longer are competitive.

The Firestone closings will raise the number of jobs lost in the industry since 1975 to 19,200, according to the rubber workers union.

The U.S. industry had made a gamble 10 years ago that belted-bias tires would be able to fend off the more expensive radials, which required a greater capital investment to produce, Haydon said, The gamble failed, and radials are expected to capture 80 percent of new tire sales this year and more than half the replacement market.

The plight of tire manufacturers now is typical of industries that banked on continuation of "cheap" energy under government controls, Haydon said. Now soaring gasoline prices put a premium on smaller cars and the more fuel-efficient radicals. "Factories producing energy-inefficient products -- like bias-ply tires -- are going to become softball fields," he said.