Richard J. Gillis, head of one of the area's largest car rental agencies, and his company pleaded guilty in Alexandria yesterday to various charges growing out of an alleged odometer tampering scheme that federal prosectors said involved hundreds of cars sold by the rental firm.

The company, which operates as Americar Rental Systems, pleaded guilty to mail fraud, obstruction of justice and four counts of odometer tampering.

Gillis, 46, a former president of the America Car Rental Association, made his guilty pleas to the 23 misdemeanor charges under an arrangement that requires him to be jailed. A proponent of tougher federal odometer laws in the past, Gillis faces a maximum of 23 years in prison and $1.15 million in fines. His company could be fined up to $206,000.

District Court Judge Richard L. Williams set sentencing for Aug. 2 and said he would impose whatever prison time and fines could be agreed upon by Gillis' lawyer and prosecutors.

A separate civil suit against the Alexandria-based firm by the Virginia attorney general's office "is close to a settlement," Gillis said in a statement yesterday. The suit was filed on behalf of 108 consumers who bought cars from Americar on which odometers allegedly had been tampered with. Charging violations of state and federal consumer laws, the state is seeking $290,000 in penalties and damages.

"An offer has been made, but it's not yet approved," said assistant state attorney general Wayne Halbleib.

The civil suit and criminal charges against Gillis and his 7-year-old company, which has rental outlets in Virginia, D.C. and Maryland, resulted from a year-long investigation by federal and state authorities into odometer tampering by Northern Virginia used-car dealers.

It is a federal offense to roll back odometers in order to enhance the value of a vehicle. Each time a car is sold the buyer must sign an odometer disclosure statement stating the true mileage reading.

A 35-count indictment last April against Gillis and his firm, Corporate Fleet Management Inc., charged that between November 1981 and April 1984 Americar sold about 1,000 used cars from its rental fleet, more than half of them with tampered odometers. The indictment listed 178 such cars.

The mail fraud count against Corporate Fleet Management referred to the mailing of deceptive odometer readings to a company in Foxboro, Mass., that issued warranties on used cars. Cars with lower odometer readings are more easily warranted.

"By personally pleading guilty, Mr. Gillis is acknowledging his responsibility as chief operating officer of the company for the actions of subordinate employes," Americar said in the statement, " . . . even though the employes, to a large extent, were acting on their own."

Assistant U.S. attorney Clarence Albright said yesterday in court that the obstruction of justice charge stemmed from conversations between Gillis and one of his former employes, Richard A. Moore, during which "Gillis told Moore to get rid of records" after they had been subpoenaed by an Alexandria federal grand jury in early 1984.

Albright said in court that if the case had gone to trial, Moore would have testified that he informed Gillis that some employes were rolling back odometers and that Gillis had replied that he knew about the actions, which he called "commonplace in the industry."

Moore, 42, who was fired by the firm, received a one-year suspended sentence and was fined $5,000 April 19 after pleading guilty to four counts of odometer tampering and one count of conspiracy to alter odometers on cars sold by Americar