In a final gesture to the suffering American auto industry, President Carter will ask Congress to add $100 million to the fiscal 1982 budget to speed up the government's purchase of automobiles, trucks and other vehicles, administration sources said yesterday.

Both Carter and President-elect Ronald Reagan proposed similar help for the auto companies during the presidential campaign, and while Carter is renewing the offer in his final budget request to Congress, it is not clear that Ronald Reagan will endorse it when he sends Congress his revised budget for fiscal 1981.

Rep. David Stockman (R-Mich.), chosen by Reagan to head the Office of Management and Budget, didn't comment on that specific proposal Thursday during his Senate confirmation hearing, but he took a dim view generally on special federal aid for ailing industries.

Congress may not be receptive, either. It didn't approve Carter's request last year to add $100 million to the current budget of the General Services Administration, which operates the government's primary vehicle fleet.

Carter's proposal would have raised the GSA vehicle budget from $71 million to $171 million, permitting the agengy to purchase 8,600 sedans instead of 6,000 and 10,500 light trucks instead of 4,000. The number of new special vehicles such as ambulances and fire trucks would have been increased from 1,000 to 2,500. A GSA spokesman said the policy did not involve the purchase of additional cars, but rather speeding up the replacement of old cars with new ones.

Stockman was challenged Thursday by several Democratic senators who contended that even if Reagan's tax and budget proposals are enacted, and even if they work, the process will take too long to do much good for auto, steel and other troubled industries. "They need immediate help," he said.

Stockman replied that the best thing the Reagan administration could do for the auto industry is to get the economy expanding at a healthy rate once again. Once auto sales return to an annual level of 12 million cars, instead of the 9 million rate currently, the auto companies will be prosperous again.