General Motors Corp. and Ford Motor Co., the nation's largest automobile manufacturers, have agreed to produce up to 440,000 cars in model years 1980 and 1981 equipped with air bags - devices that inflate instantly during a crash and protect drivers or passengers.

In addition, Mercedes-Benz has agreed to manufacture 2,250 air-bag-equipped cars and then 60,000 cars equipped with passive seat belts that lock upon crash, Secretary of Transportation William T. Coleman Jr. announced yesterday.

Three insurance firms - Allstate, Nationwide and Volkswagen Insurance - have agreed to continue offering premium reductions to owners of cars equipped either with air bags or passive belts.

Coleman had called for such a demonstration program Dec. 6, in deciding not to mandate air bags or passive seat belts on all new cars. He expressed the fear that many Americans would oppose a mandatory program although he concluded that the passive restraints would prevent many deaths and were technologically and economically feasible.

In announcing the industry's agreements yesterday, Coleman noted that auto company officials had convinced him of the need to delay implementation because of production schedules. Thus the proposed demonstration will begin one year later than Coleman requested.

Coleman also emphasized that Congress and the incoming administration could decide at any time to mandate installation of air bags. "Nothing I did binds a future secretary from mandating passive restraints as his very first act," Coleman asserted. The contracts signed with GM and Ford provide for an immediate end to the demonstration at any time a mandatory program is proposed by the Department of Transportation.

Secretary-designate Brock Adams has not indicated what course the incoming administration will take on air bags. At his Senate confirmation hearing, Adams said the Coleman decision was "a subjective judgment . . . which must be reviewed most carefully."

Among terms of the contracts announced yesterday: