The Firestone Tire and Rubber Co. has agreed to recall another 5 million steel-belted radial tires because of possible defects, the government announced yesterday.

The action is an expansion of the company's agreement in November 1978 to recall 14 million Firestone 500 steel-belted radials and related TPC and private-brand tires. That was the largest such recall in history.

The tires involved in this recall are also Firestone 500s, as well as private brands of identical construction made by Firestone and sold by Montgomery Ward.

The company said in a statement it "continues to believe that the tire lines do not contain safety-related defects," and that it is taking the new action "primarily to improve consumer confidence" in Firestone products.

Firestone estimated 1.8 million of the 5 million tires to be recalled are still in service. The company will notify owners by mail and offer free replacement of tires sold after June 1, 1977. Most will be replaced with new Firestone 721 steel-belted radials or other comparable tires.

Replacement at half price or on a pro-rata price based on treadwear will be offered to consumers who purchased their tires before June 1, 1977.

The tires involved in the new recall were produced in 1976 and 1977, and most are size "G" or larger. They generally are found on intermediate- to full-size sedans and station wagons.

Firestone, which has been hit hard by adverse publicity over the massive recall, said dealers would be available to answer questions on the new recall on or after July 7. Consumers also may obtain additional information by calling the Firestone Consumer Affairs Office toll free at 800-321-9638.

"We are pleased with Firestone's conscientious response to the newly developed evidence that some additional tires from the 500 lines should be included in the recall," said Joan Claybrook, head of the National Highway Traffic Safety Administration.

"The company's decision to take these tires off the highway is a commendable and responsible action. We urge all owners of the affected tires to participate in this important safety campaign," she said.

The company recall two years ago was done at the urging of the NHTSA, which acted on reports that 41 deaths and 65 injuries allegedly were caused by blowouts or other failures of the tires.

The cost of the additional recall is expected to be covered under the $234 million set aside in 1978 for the original recall. The recall resulted in an after-tax loss of $147 million, charged to the final quarter of 1978, the company said.

The company, in the midst of a campaign to enchance its image following the 1978 recall, said in March it was extending the original recall indefinitely. Free replacement of the tires recalled in 1978 will continue until Oct. 1, after which affected tires would be replaced on a pro-rata treadware basis, the company said.