General Motors Corp. is accusing the National Highway Traffic Safety Administration of rigging a televised test showing the rear-wheel assembly falling from a 1980-model, intermediate GM station wagon, according to a GM letter obtained by The Washington Post yesterday.

The GM letter said that NHTSA removed a key part from the wheel assembly of a test car, drove the car around a test track and then filmed the predictable accident.

NHTSA announced on April 1 that it had found a potentially "catastrophic" defect in as many as 5.3 million midsize GM cars and sedan pickups produced from 1978 through 1980.

NHTSA charged that an improperly manufactured retaining part could fail, causing the rear axle shaft and rear wheels to work loose and possibly separate from a car, "which could lead to loss of control, accidents, injuries, death or property damage."

The dramatic NHTSA film, which was broadcast April 1 by some of the television networks, showed the rear-wheel assembly separating from a 1980-model GM wagon, supposedly as the result of the alleged defect.

NHTSA said that a part called an end button could work lose in some cases, permitting a "C-lock" retaining ring to drop off, resulting in excessive movement of the rear axle shaft.

GM said in its letter to dealers that NHTSA deliberately removed a C-lock from the test station wagon to simulate the results of rear-axle parts failure. The NHTSA film did not identify the test as a simulation.

"As you may have seen recently on TV, a test film distributed by NHTSA showed an axle separation which was simulated by removing a retaining part," the GM letter said. Similar rear-wheel assemblies are used by other manufacturers and the axles also would have separated from their cars had the retaining clips been removed, GM said.

The company has complained to NHTSA that the announcement "created unwarranted and unnecessary fears" among GM customers.

An NHTSA official said in an interview last week that the test was not a simulation.

"It was an actual test," the official said. "It was done at our testing ground in East Liberty, Ohio," the official said. Asked how NHTSA engineers happened to film, at the exact time of breakdown, one of the 5.3 million cars that might have the defect, the NHTSA official said: "In a lot of cases, we film our tests. In this particular case, we had good footage."

However, NHTSA Administrator Raymond A. Peck Jr. said yesterday that he would withhold comment on the letter "until we have it in our possession and can respond appropriately."

GM officials said yesterday that NHTSA admitted in a letter to the company that the televised test example was a simulation.

"They explained it in their own paperwork . . . They said that they did pull the C-ring," GM spokesman John Hartnett said yesterday. Hartnett said GM officials requested the NHTSA film and accompanying documentation in an attempt to determine how the test was done. "How many millions of feet of film do you have to shoot before you get the rear axle on one of those cars coming off?" Hartnett asked.

GM officials attempted to answer that question in the corporate letter to dealers:

"It has been estimated that the 5.3 million General Motors U.S. vehicles cited by NHTSA were driven more than 50 billion miles in the past year. In all of these miles, there have been reports of 67 incidents of axle malfunction.

"In only 20 of these incidents, the wheel and axle separated from the vehicle with one alleged injury accident involving muscle strain," the GM letter continued. "This data translates into one such separation for every 265,000 years of vehicle use. Such an extremely low rate puts into perspective how remote the possibility is that a customer will ever experience this type of incident."

GM spokesman Clifford Merriott said "NHTSA has been talked to." No further action is planned at this time, he added.

NHTSA said it will hold a public hearing on its finding, called an "initial determination of defect," on May 4. The hearing is scheduled to begin at 10 a.m. in Room 2230 of the Department of Transportation headquarters, 400 Seventh St. SW.