A federal grand jury in Alexandria is investigating allegations of odometer tampering by Northern Virginia used car dealers, a practice federal officials called widespread and believed to involve hundreds of automobiles.

Officials from the U.S. Department of Transportation and the Virginia Division of Motor Vehicles have seized records from at least six used car dealers in the area during the last year, according to search warrants filed in federal court. One person, a mechanic from the District, has been charged with violating a federal odometer tampering law and will appear in court on the charges today.

"We've got an investigation into odometer tampering all over Northern Virginia," said Assistant U.S. Attorney Robert Cynkar, confirming a report carried by WJLA-TV.

Odometer tampering "became widespread in the last three to four years because car prices and interest rates went sky high," said Richard C. Morse, chief of the odometer fraud staff at the National Highway Traffic Safety Administration. "In l981 and l982 used cars became in great demand, but people wanted low-mileage used cars," he said.

Morse said his agency, part of the Transportation Department, has been supporting the Virginia DMV and Virginia State Police, who "are doing most of the investigating."

The only person charged in connection with the federal investigation is Reginald Guy, a mechanic who lives in Southeast Washington. Guy, arrested in January, was charged with six counts of odometer tampering and conspiracy to tamper odometers that allegedly occurred in Arlington between February and April of last year.

Last April records were taken by officials from Americar Rental Systems in Alexandria and from Thrifty-Rent-A-Car in Springfield, according to court records. An affidavit signed by Morse in support of the search warrants said that an official of Virginia's DMV had received information from an informant that odometer tampering was going on at Americar. The affidavit also said DMV had put the premises under photographic surveillance and had photos "that appear to be of a person engaged in the act of rolling back the odometers of cars."

An Americar spokesman said that an internal investigation had found that some employes without the knowledge of the company owners had "violated the federal odometer statute." Those employes were discharged early this year, a spokesman said. A spokesman for Thrifty could not be reached.

"We're told that Americar is not a target of the investigation," said William Cummings, an attorney representing the company. Cummings said his clients were cooperating with the investigation. One company official testified before the grand jury yesterday.

Last August four used car dealers in Arlington were asked to turn over documents and records. In one case, Ballston Auto Sales owner John Lyon said that about half the papers seized had been returned to him.

Other used car dealers from whom records were seized include Fairfax Cars, Fairfax Auto Brokers and Major Motors in Arlington. Fairfax Cars owner Jim Eckels said, "I'm trying to run a good honest business; somebody has been buying and selling cars using our name."

A person who identified himself as manager of Fairfax Auto Brokers declined to comment. Officials of Major Motors could not be reached for comment.

Under federal law every time a car is sold the buyer and seller must sign an odometer disclosure statement stating the correct mileage on the vehicle.