A Firestone Tire & Rubber Co. quality assurance official warned his superiors last April that the field performance of several Firestone tire models-besides the troubled "500" steel-belted radial-had "slipped," and the company could be complicating its problems by making several production changes with little or no prior testing, according to documents released by the federal government.

Firestone quality assurance staffer R. W. French, in an April 18, 1978, memo to his boss, W. E. Moore, said that "many measures of field performance indicate our tires have been below Firestone standards."

"The steel radial 500 is the most outstanding example of this," French added, "although other lines have slipped."

French said in his memo that although the "721" steel-belted radial-the tire that has replaced the discontinued "500"-"is a substantial improvement, Firestone's reputation with customers is still at an all-time low ebb."

The "721" is the top-of-the-line steel-belted radial tire that is being used to replace, free of charge, the millions of "500" steel-belted radials now being returned to Firestone by consumers under the largest product recall in history.

Firestone has refused to comment on the release of its internal documents by the government, except to say that "We ddo not think any purpose is served by post-mortem discussion and dissection of isolated documents."

Firestone executives consider the "721" to be a far superior tire to the "500" and are selling the "721" with an unprecedented two-year warranty in an effort to regain at least some of the consumer confidence the company lost in connection with the "500" recall.

The National Highway Traffic Safety Administration, citing thousands of failures leading to dozens of deaths and injuries, negotiated and signed the Firestone "500" recall-which is expected to cover 7.5 million tires and has already resulted in the company replacing 750,000 tires and taking a $234 million tax write-off.

Late last week the NHTSA released hundreds of paces of previously confidential Firestone documents which revealed that the company, contrary to its earlier statements, had knowledge of problems with the "500" as early as 1972, only a few months after the tire was first produced.

But the 1978 quality assurance memo is the first indication of concern within the company over the quality of other Firestone tires, including the much-heralded 721. Consumer groups, notably the Center for Auto Safety, which was instrumental in providing early indications of problems with the "500," have maintained that it is still too early to tell if there are any problems with the 721.

The Firestone documents don't indicate that such problems necessarily exist, but do express concern that the company is not adequately testing changes made in the tire.

"A prudent corporate position at a time like this." French said in April, "would be to thoroughly test any changes made to the product prior to production to assure the changes being made will not inadvertently cause performance problems.

"With the above in mind, we note a large number of changes going into production with limited test background, no U.S.A. test background, or no test background at all.

"These changes," French concluded, "affect large portions of all production and also our reputation."

French then listed five specific production changes made for various Firestone tires. In three of these cases, he wrote "Tested in Europe-No U.S.A. Test." In one case he wrote "No testing." And in the fifth case he wrote, "One successful U.S.A. test. . . many unsuccessful tests."

French stressed that the "probability that any of the above items will cause a major field problem is low, but I do not believe that in our present product situation we can take even that small risk."