The National Highway Traffic Safety Administration (NHTSA) yesterday formally proposed loosening federal standards that require cars to be equipped with bumpers capable of sustaining 5 mph collisions without damage to the vehicles.

The agency instead proposed options ranging from keeping the 5 mph standard on the front bumper but reducing it to 2 1/2 mph on the rear, to eliminating the standards altogether. Public hearings are scheduled to begin in three weeks with a final ruling expected early next year.

NHTSA chief Raymond Peck, a Reagan appointee, and the automobile industry have condemned the existing bumper standards as excessive regulation that probably results in higher costs for consumers. The rules were phased in beginning with 1973 model cars in an attempt to reduce costs of repairs caused by minor accidents.

"We can save the consumer more money by changing the standard," said NHTSA spokesman Hal Paris. According to an agency study, the regulations have tended to be cost-effective for front bumpers, where most collisions occur, but not so for rear bumpers, suggesting that revision is needed.

NHTSA's proposals were challenged yesterday by the Center for Auto Safety, part of the Nader organization. "We are going to oppose this at every step of the way," said Clarence Ditlow, the center's director. " . . . I think you'll see a consumer revolt over this one."

Ditlow said the proposals could bring back the days when minor scrapes in parking lots routinely resulted in repair bills of $200 to $300. Loosening the standards "can only cost the consumer more money," he said. "The benefits of five-mile-an-hour bumpers greatly exceed the cost of producing the bumpers."

Rep. Tim Wirth (D-Col.), chairman of a House subcommittee dealing with consumer protection, said in a statement that it was unclear if NHTSA's options would save money for consumers. He cited testimony before his subcommittee that it would cost the auto industry $30 million to retool from 5-mph bumpers.

The Motor Vehicle Manufacturers Association, which represents the big Detroit auto firms, has sought revisions in the standards for years because of the costs. Last February, its board wrote to President Reagan urging that the standards be relaxed to 2 1/2 mph for both front and rear bumpers.

These bumpers would be cheaper to produce than 5-mph bumpers and would also lower vehicle weight, according to association official Tom Carr. The association believes that consumer savings on purchase price and fuel would outstrip savings resulting from the 5-mph bumpers, Carr said.

Insurance companies have lobbied strongly for keeping existing standards. State Farm executive Donald McHugh told hearings convened in July by Wirth's panel that "over the last decade the standard has served to eliminate entirely hundreds of thousands of small claims that our policyholders would otherwise be filing."

For older cars not covered by the standards, he said, "almost half of the damage claims reported to State Farm involved . . . speeds of 5 mph or less."