The National Highway Traffic Safety Administration often takes years to complete investigations of possible safety problems on automobiles while owners are driving the potentially unsafe cars, a new General Accounting Office report says.

Further, the congressional auditors said, only about half of the automobile owners who receive recall notices bother to get safety defects repaired, possibly because recall letters are so difficult to understand. The GAO said a postcard campaign following up the letters would result in more cars being fixed.

"Statistics indicate that the longer it takes to initiate a recall, the less owners are likely to respond by having their vehicles repaired," the GAO said.

It found, for example, six cases in which NHTSA investigations lasted between 67 and 86 months. When recalls were finally issued, the vehicles in question were 5 to 19 years old; only between 8 percent and 20.5 percent of the owners contacted responded to the recall notice.

Since the federal program started in 1966, about 97 million vehicles, 6 million replacement items and 25 million tires have been recalled.

Raymond A. Peck Jr., NHTSA administrator, said yesterday that, while GAO's numbers were accurate, he feels it is unfair to cite statistics that include cases where NHTSA had to sue the manufacturer to start a recall.

In the last year of the Carter administration, according to both industry and government sources, NHTSA's approach turned much less adversarial, and voluntary recalls have been more frequent.

"The public is served best when, even in cases of doubt, the companies recall voluntarily," Peck said. "A litigation is a failure because it means somebody is driving around out there" with a potentially unsafe automobile.

For all the recalls between 1966 and 1979, the GAO said, an average of 53.5 percent of those contacted brought their cars in. That includes 55.4 percent of the owners of domestic automobiles and 40.6 percent of the owners of foreign cars.

One difficulty with such statistics, Peck said, is that they have no way of showing how many older automobiles have changed ownership or have been junked. In those cases, the recipient of a recall letter simply tosses it into the wastebasket, which lowers the response percentage. Response rates for newer models, Peck said, are as high as 85 to 90 percent.

A majority of recalls are initiated by the manufacturers. NHTSA told GAO that 15 to 20 percent of all recalls were initiated because of NHTSA involvement, but those recalls account for 50 to 70 percent of the total number of vehicles recalled.

The GAO had a good deal of criticism for the typical recall letter. GAO's consultants said that 54 percent of U.S. adults read at or below the 11th grade level, while the usual recall letter requires college reading levels (12.4 to 16.4 years of school).

Some of the paragraphs in the letter are dictated by regulation, but with them are sentences like this:

"Loss of front braking at a time when minimum stopping distance is required could result in vehicle crash without prior warning."

The GAO consultant suggested this: "Without front brakes, you could crash if you need to stop in a short distance."

Peck said he would study that idea as well as the suggestion of a follow-up postcard to improve responses. However, he said, NHTSA statistics show that people receiving the first notice often "made the wrong judgment as to how to treat it. We have not found that people do not understand what it is all about."

Chet Womack of General Motors said GM sends a second letter 60 days after the first for those who have not come in. He said that in a recent GM recall of 5.8 million mid-sized vehicles for a rear suspension problem, 77.3 percent had been repaired after 16 months. "This was a voluntary recall," Womack said. "NHTSA didn't know anything about it until we advised them."

A spokesman for Ford had a similar reaction to the postcard suggestion. Four to six months after the first recall notice is sent, he said, Ford sends a second letter to those who have not responded, then urges dealers to locate the owners and encourage them to come in.