The president of Americar Rental Systems, who just finished a term as president of the American Car Rental Association, was indicted yesterday in federal court in Alexandria on charges of tampering with the odometers of 178 cars sold by Americar during a 2 1/2-year period.

In addition, the Virginia Attorney General's Office filed a suit seeking more than $290,000 from the company on behalf of consumers. Americar, one of the area's largest car rental agencies, has a large sales operation to dispose of its used cars.

The 35-count indictment against company President Richard J. Gillis also charges him with mail fraud and obstruction of justice. The Alexandria-based Corporate Fleet Management Inc., the umbrella company for the Americar rental outlets in Maryland, D.C. and Virginia, also is named as a defendant.

The civil suit against Americar, said to be one of the largest ever filed as a result of odometer tampering, charges that the company violated state and federal consumer laws.

Used car dealers are required to keep records of the mileage on cars they sell for four years. It is a federal offense to roll back odometers to enhance a car's value.

Gillis was out of town yesterday, but said in a statement that he has been a proponent of tougher odometer laws and was preparing to testify before Congress on behalf of such laws. If convicted, Gillis faces maximum penalties of 83 years in prison and $1.2 million in fines.

Gillis said that he was "astounded" by the indictment, which he blamed on "the fraudulent and malicious crimes of past employes and coconspirators outside the company."

Gillis said that employes of Americar, which was founded in l978, were altering the odometers of cars without his knowledge for their own personal gain. He said that after an internal investigation, the employes were discharged.

One former employe of Americar, Richard A. Moore, was given a one-year suspended sentence and fined $5,000 on April 19 after pleading guilty last month to four counts of odometer tampering and one count of conspiracy to alter the odometers of cars sold by Americar.

Gillis was named as a coconspirator in Moore's indictment.

The indictment and civil suit are the products of a year-long investigation by federal and state authorities into odometer tampering by Northern Virginia car dealers.

The civil suit demonstrates "the seriousness with which we take this problem," said Attorney General Gerald L. Baliles in a statement. "Rollbacks victimize automobile purchasers, legitimate dealers and the automobile industry."

A spokesman for the attorney general's office said that consumers would be notified if the suit is successful. He said other damage claims may be filed and there may be more civil suits against other Northern Virginia car dealers.

The $291,600 in damages the suit seeks includes $1,500 for 108 consumers who unknowingly purchased cars whose odometers had been tampered with between l981 and l984; $108,000 in civil penalties and $21,600 for investigative and legal costs.

Joan C. Skilling of Alexandria said she would put the money aside for a vacation if she receives damages for the 1980 Buick Century she bought from Americar. "I didn't think that sort of thing went on anymore," she said of the tampering.

The indictment charges that about 1,000 used cars were sold by Americar between November l981 and April l984, and that more than half of them had their odometers rolled back.

The 11 mail fraud counts refer to Gillis' alleged mailing of the deceptive odometer readings to a company in Foxboro, Mass., which issued warranties on the used cars.

The obstruction of justice charge relates to an allegation that Gillis ordered Moore, who was them working at Americar, to destroy rental records that had been subpoenaed by a federal grand jury.

Gillis is scheduled to be arraigned Tuesday.