The consumer group that triggered the recall of millions of Firestone 500 steel-belted radial tires charged yesterday that the tires used to replace the defective 500's are just as dangerous.

The Center for Auto Safety filed a petition asking the National Highway Traffic Safety Administration to open a new investigation of alleged safety defects in the Firestone 721 steel-belted radial, and all Firestone 500 and TPC steel-belted radials manufactured after May 1, 1976 -- the last date covered by the existing recall agreement.

Citing new accident and tire failure data, the Center said in a letter to NHTSA administrator Joan Claybrook that accompanied the petition that it "has been contacted by consumers with hundreds of failed Firestone 500 and related steel-belted radial tires that were made after the cutoff date for the recall of the tires."

Claybrook said in an interview that her agency's tire-failure data "doesn't seem to indicate a basis for the opening of an investigation." If there was a need for another probe, "we would have done it," she said, adding, however, that NHTSA would look into the Center's charges.

Claybrook said the number of post-recall failures "looks comparatively small."

In a telephone interview from Akron, a Firestone spokesman said, "We consider Mr. Ditlow's (Clarence Ditlow, executive director of the Center) criticism of the recall agreement unjustified and believe that his petition is without merit. The entire 500 picture was thoroughly reviewed by NHTSA and Firestone during the discussions that led to the recall. It was only after that review that the cutoff dates were established."

The spokesman said there are 21 million 721s in use today, and "from the overwhelming reaction of our customers, we are confident (it) is the finest radial on the road today."

The Center's petition states that it has received 190 letters and telephone calls "about specific instances of defective performance in over 300 tires made by Firestone after the recall period.

"Failure modes include blowouts, tread separations, slipped belts, flats, splits and excessive wear," the Center said. It added that many of the failures were similar to those incured in tires covered by the recall.

The consumer group said that 17 percent of all of the 24 million Firestone 500s produced were built after May, 1976.

In its own survey of failed 500s, the Center said, virtually the same percentage of the 406 failed tires were made after the same date.

"Analysis of the Firestone complaints relating to tires not covered by the recall reveals that the non-recalled tires show the same failure modes and subject their owners to the same risks as do the tires recalled," the Center concluded.

The filing indicated that one reason NHTSA may have not acted on the later tires was the long time delay between manufacture and sale of tires.

"Since tires frequently are not sold until several years after manufacture, Firestone tires made from Ay 1, 1976 through 1977 are just beginning to get substantial use," the Center said.

The Center further claimed in its letter to Claybrook that "for years, Firestone has experimented with changes in construction, which were put into production without any knowledge that the changes would improve the shoddy performance of the product."

The letter cited an April 18, 1978 internal Firestone memo, portions of which have been published in The Washington Post, in which a Firestone quality assurance division executive wrote, "(We) note a large number of changes (in several tire lines) going into production with limited test background, no U.S.A. test background, no test background at all."

The executive speculated that many tire lines may have slipped in quality.

NHTSA data shows that of more than 14,000 complaints received by May 21, 1979 concerning more than 38,000 Firestone 500 tire failures, only 182 complaints covering 275 failures involved post-recall tires.

Claybrook said the agency is still evaluating the 251 additional complaints it has received on Firestone tires since May, as well as the additional 190 submitted by the Center yesterday to see if they reveal anything significant.