WASHINGTON (AP) _ A consumer group on Friday accused the government of keeping the public in the dark about the comparative durability of tires, criticizing the Transportation Department's failure to publicize tire treadwear test results.

The National Highway Traffic Safety Administration, which is part of the department, reinstituted its tire-grading requirements last December under a federal court order.

The Center for Auto Safety, a group founded by Ralph Nader, complained Friday that while the test results were supposed to help consumers the agency ''has buried the invaluable treadwear ratings deep in the bowels of a regulatory docket where no consumer is sure to go.''

Judy Petty, a spokeswoman for the highway safety agency, denied any suggestion that the test results are being hidden from the public and said the tire comparisons are ''available to anybody that wants them.''

''We don't send them out as a press release because its just page after page after page of numbers, but they are available to anybody that wants a copy of them,'' she said.

The tests, most of which were conducted during the winter, compare treadware among the numerous tire brands and types that are available. Consumer advocates long have argued that such a comparison is needed to give customers a way to compare the durability of one tire from another and determine what they are getting for their money.

Tires with high treadwear often cost considerably more than those with a low treadwear rating.

Beginning in September, tire manufacturers will be required to mold the test results - displayed as numerical rating, with the higher numbers showing longer treadwear - onto each tire.

The Center for Auto Safety obtained from the NHTSA files and released the comparative treadwear of 134 radial tires sold by 18 companies.

The top performers were three Michelin tires, all of which had ratings of more than 300. The top-rated tire, a Michelin XH, with a rating of 330, is expected to have a treadwear of 66,000 miles under normal driving conditions.

Another Michelin, the XWX, did relatively poorly, however, with a rating of only 100. At the bottom of the list was the Bridgestone 147V-70 with a rating of 80, equivalent to a life of about 16,000 miles.

Most of the tire manufacturers had strongly opposed the tire rating regulations when they were imposed by the Transportation Department in the late 1970s under the Carter administration.

The regulations were suspended after the Reagan administration took office, but in April 1984 the U.S. Court of Appeals ordered them reimposed.

One tire manufacturer, Uniroyal, had been a strong supporter of the treadwear testing. In the latest test, Uniroyal did comparatively well, with several tires including its Tiger Paw Plus models scoring in the 260 to 280 range, meaning a treadwear of 52,000 to 56,000 miles.

Under average driving conditions, a tire with a treadwear rating of 100 should get 20,000 miles, while one with a rating of 250 should get about 50,000 miles, according to government and industry estimates.