The National Highway Traffic Safety Administration has deflated the claims of some tire manufacturers who have been using "federal" treadwear grades to tout the durability of their products.

The NHTSA, responding to complaints about competitors by Goodyear Tire and Rubber Co., said that advertisements citing the grades as proof that certain tires are "best" in terms of treadwear could be misleading.

The NHTSA letter was written by agency Chief Counsel Erika Z. Jones last month to Goodyear attorneys who had complained that some competitors were misusing the Uniform Tire Quality Grading Standards.

The standards are meant to give consumers an idea of how many miles certain tires can travel before they need to be replaced.

Former NHTSA administrator Raymond Peck rescinded the standards in 1983 on the grounds that they were not reliable. But in response to petitions brought by consumer advocacy groups, including the Washington-based Center for Auto Safety, Peck's action was reversed by a federal appellate court here. The standards were reinstated Dec. 15, 1984.

The rubber wars resumed soon afterwards.

Manufacturers whose tires apparently fared well under the grading system took out national advertisements in newspapers, magazines and other media. And much to the chagrin of companies like Goodyear, there also were news reports describing competitors' tires as "best," based on information contained in the treadwear reports.

But Goodyear's attorneys argued that the treadwear reports omitted other important information -- such as tire design and intended use, the effects of vehicle maintenance and individual driving habits on tire wear, the kinds of roads over which tires are driven, and the kinds of climates in which they are used.

The NHTSA chief counsel agreed with Goodyear's complaints.

For one thing, Jones said in her letter, the tests used to get the grades "were conducted entirely by the individual tire manufacturers, not by the federal government."

The NHTSA publishes the tire-grade information after it receives it from the manufacturer, Jones said. The validity of the information is open to question, and misinterpretation, because of the variety of tires on the market and because the tests are conducted under laboratory conditions rather than on the road, she said.

"Neither tire manufacturers nor the agency made, or could make, any total tire mileage projections from the reported treadwear grades," Jones said. "If one were to project total mileage from the treadwear grade, one could say only that a certain tire might get 'x' miles if driven over the same roads at the same speeds on the same vehicles with the same careful maintenance performed daily on those vehicles.

"A projection of a tire's tread life which did not include all of those caveats would be misleading."

She added: "The agency does not categorize particular tires as the 'best' or the 'worst' based sole on the treadwear grades assigned by the manufacturers."

Clarence Ditlow, director of the Center for Auto Safety, said yesterday that he was satisfied that the U.S. Circuit Court of Appeals here thinks the treadwear standards are fair.

"The court has upheld our position," Ditlow said. "NHTSA has been trying to kill the treadwear standards for years. It doesn't surprise me that the agency is now in bed with the companies" on this issue.

COLLECTIONS . . . The U.S. Treasury is in line to pick up $6,015,990, thanks to the NHTSA. The money represents the fine Britain's Jaguar Cars Inc. has agreed to pay to the U.S. government for violating the NHTSA's 1983 and 1984 fuel economy standards. Jaguar, so far, is the only company to be fined.