A former employe of one of the Washington area's largest car rental agencies pleaded guilty in U.S. District Court in Alexandria yesterday to charges of tampering with the odometers of cars sold by the company, and the firm's president was named by prosecutors as a co-conspirator.

Richard A. Moore, 42, formerly fleet manager at Americar Rental Systems, pleaded guilty to a misdemeanor charge of conspiring with Americar President Richard Gillis and others to roll back the mileage on cars from late 1981 through April 1984. Gillis was not named as a defendant in Moore's preindictment plea.

The charges stem from a year-long federal grand jury investigation into allegations of odometer tampering, a practice federal officials have called widespread and believed to involve hundreds of automobiles.

Federal prosecutor Robert J. Cynkar said he has no way of knowing how many cars were sold with tampered odometers. "It's lots," he said.

Under federal law every time a car is sold both the buyer and seller must sign an odometer disclosure statement giving the correct mileage of the vehicle. Dealers are required to keep such records for four years.

Moore also pleaded guilty to four misdemeanor counts of arranging for the odometers of four cars to be turned back. Facing a maximum sentence of five years in jail and fines of up to $50,000, Moore was released on personal recognizance until his sentencing on April 19.

Asked why Gillis was not charged as a defendant, Cynkar said "our case isn't over yet."

"I'm a little surprised," Gillis' lawyer, William Cummings, said of the information presented in court yesterday.

"Moore had told me there had never been any rollbacks, nor had Mr. Gillis directed or approved any improper activity . . . . It may well be the actions of disgruntled employe."

In a statement yesterday, Gillis said: "The employe who committed the illegal act has pleaded guilty to the charges and is attempting to shift the focus of the blame from himself to the company."

Last month Americar officials said an internal investigation found that some employes had been tampering with odometers without the knowledge of Gillis and other company employes. The employes were discharged, they said.

Moore was discharged from his job in early February.

Moore is the second person to plead guilty to charges in connection with the federal investigation. Reginald Guy, a District mechanic, pleaded guilty Feb. 27 to six misdemeanor counts of odometer tampering and conspiracy to tamper. He faces up to a year in prison and a $50,000 fine and will be sentenced March 28.

According to information presented by government prosecutors, Guy altered the odometers on four cars owned by Americar between February and March last year at sites in Arlington.

Prosecutor Carroll Weimar said the government had copies of canceled checks from Americar signed by Moore that show the company paid $45 for each odometer rollback.