Many auto makers selling cars in the United States are putting bogus five-mile-per-hour bumpers on new cars, a group of auto safety advocates contended yesterday.

True 5-mph bumpers are supposed to protect car bodies from property damage at crash speeds of 5 mph or less. The allegedly bogus bumpers "are so weak, they allow huge repair costs after 5-mph impacts," the Washington-based Insurance Institute for Highway Safety said.

"This is a tremendous racket," said Clarence Ditlow, director of the Center for Auto Safety in Washington. "Some car manufacturers are putting on 2.5-mph bumpers, some are putting on strong 5's and many others are putting on weak 5's," Ditlow said.

The "weak 5's amount to deception," he said. "They might protect the car, but they disintegrate in a low-speed collision and the customer still winds up paying $200 for a 20-minute repair job," Ditlow said.

General Motors Corp., the nation's largest auto maker and one of the prime targets of the safety groups' criticism, had no immediate response to the charges. But a spokesman for Ford Motor Co. said yesterday that his company stands behind its bumpers.

"Ford Motor Co. meets the essential criteria of federal bumper standards," the Ford spokesman said. "Our bumpers do not allow any damage to the car" at 5 mph. "But you might get a dimple or a scratch on the bumper itself."

At issue is the National Highway Traffic Safety Administration's May 1982 rollback of a standard requiring all cars sold in the United States to be equipped with 5-mph bumpers. NHTSA said then that the reduction of the standard to 2.5 mph would save consumers an estimated $300 million annually in vehicle costs, largely because of fuel savings resulting from lighter bumpers. None of those savings has been realized, critics say.

A U.S. Court of Appeals panel here upheld NHTSA's ruling last month. But opponents contend that the court erred by not ruling on the validity of the agency's reasoning for relaxing the rule.

The court also failed to give NHTSA any directions for policing bumper standards -- an oversight that has produced 5-mph protection claims that don't hold up in crashes, opponents say.

State Farm Mutual Automobile Insurance Co. and Allstate Insurance Cos., two of the leading critics of the weaker standards, yesterday filed a petition for a review of the case by the full Court of Appeals.