As much as 10 percent of the $200 million worth of automotive parts and supplies charged each year to U.S. government credit cards used by government drivers is estimated by the General Services Administration to be fraudulent.

The credit cards, which GSA issues to the drivers of the government's fleet of 420,000 cars and trucks, are supposed to be used only for gasoline, oil and emergency repairs.

But the study, prepared by GSA using Standard Oil Co. of Indiana data, found that new sets of tires were being purchased for the same car every few weeks, 40-gallon purchases of gasoline were being charged for cars whose tanks hold only 20 gallons, and batteries that could not possibly be used in certain vehicles were purchased anyway.

The study prompted a GSA inviestigation into abuses of credit card purchases. At least 25 alleged instances of credit card misuses are being investigated in the Washington area alone, according to sources with knowledge of the probes.

The investigations into credit card misuse comes at a time when a series of other alleged scandals are under investigation at GSA, which spands $4.5 billion a year to provide office space and supplies for government workers.

In Washington, sources say that federal prosecutors have already found that some managers of federal buildings operated by GSA have been approving little or no repair work on the buildings.

In return, sources familar with the probe say, the building managers receive cash and other favors from the contractors.

Federal prosectors in Baltimore, meanwhile, have found that some managers of GSA centers that supply U.S. workers with office supplies have been certifying they have received goods they never in fact received, sources say.

In return, according to the sources, the managers of the supply centers received free suits of clothing, color television sets and stereo equipment from the wholesale firms that provided supplies.

GSA Administrator Jay Solomon yesterday canceled a press conference that was to have been held today at which he was to discuss the investigations. No explanation was given for the cancelation.

Yesterday GSA declined to allow a reporter to interview administration officials familiar with the investigation into credit card fraud. Instead, the agency made available a spokesman who said any fraudulent purchases would reoutinely be detected in normal checks of credit card purchases.

"If a set tires is purchased, the motor pool manager would chastise the individual responsible," said the spokesman, Richard A. Dockus, who explained that a new set of tires would not be considered emergency repairs that can charged to the credit cards.

"If it was two sets purchased in a short time, we would bill the (federal) agency (employing the driver)," he said. "This is the way it happens."

Sources familiar with the investigation said GSA rarely, if ever, attempts to match credit card charges with the vehicles the driver holding the credit card was driving at the time the charges were made.

"No comparisons are made," a source said.

The result, the sources say, is that an employe can make an almost unlimited number of purchases without being detected.

Using a computer, Standard Oil Co. of Indiana this year matched each of the federal credit card purchases to the vehicle that supposedly received the parts or supplies charged. The computer survey was made in GSA regional areas with offices in Denver, Kansas City, and Chicago. GSA's estimate that 10 percent of the purchases were fraudulent was based on a comparison of the purchases with what standard fuel consumption and servicing requirements would be for each vehicle.

As a result of the investiagtions, a U.S. Army Corps of Engineers employe in Detroit was indicted last month. The indictment alleges the employe, fraudulently used a GSA credit card beginning in 1973.