When the State of Maryland goes shopping for cars, it thinks small and buys compacts and subcompacts to save money. So do Montgomery, Prince George's, Fairfax and Arlington Counties.

But the normally frugal State of Virginia still thinks big, choosing mostly midsize cars that cost more to buy and to drive.

Last fall, for example, Maryland bought 332 subcompact Chevettes that cost $4,138 each at the fleet rate and get 25 miles to the gallon. It also bought 68 subcompact Ford Fairmonts that cost $4,590 each and average 22 miles a gallon.

At the same time, Virginia bought only 100 Chevettes -- paying $537 more each for air conditioning. It selected 220 intermediate Dodge Diplomats that cost $5,612 each and get about 17 miles per gallon, and 220 midsize Plymouth Volares that cost $4,948 and get the same mileage.

The result: Virginia paid a half-million dollars more in purchase costs (and will pay more than $200,000 annually in fuel costs) than if it had followed Maryland's example.

The potential for savings in a fuel-efficient fleet can be considerable. Virginia has a nonpolice fleet of about 2,500 cars that average about 15 miles per gallon. If the state pushed its cars' average mileage per gallon to 20, the savings -- at today's gasoline prices -- would be close to $1 million, according to available figures.

"We're trying to find the most economical transportation for our people when they have to drive on state business," said Olin Broadfoot, fleet administrator with Maryland's Department of Budget and Fiscal Planning.

In Virginia, it's a different story. State employes "don't like the subcompacts," said Charles B. Walker, Virginia's secretary of administration and a member of the committee that decides what kinds of cars will be bought.

Walker said state workers prefer the roomier midsize and intermediate cars -- models that most other jurisdictions have quit buying altogether because of the steeply rising price of gasoline.

"We're buying all compacts and subcompacts -- absolutely," said Frederick I. Hiller, chief of the equipment division in Arlington's public works department.

Even police cars, Hiller said, have been scaled down from the Plymouth Fury to the smaller intermediate Chevy Nova, cutting operating costs by two to four cents a mile.

When Prince George's County Executive Lawrence J. Hogan assumed office last January, he decreed that all nonpolice county cars be subcompacts, according to James E. Doremus, acting chief of procurement and material management for the county government. But because of a budget freeze, the county has only bought four new cars, all of them Chevettes.

Sinced 1973, Montgomery County has moved from eight-cylinder cars to six-cylinder ones and, this year, to four-cylinder cars. It has kept the larger Plymouth Volare for its 600 police cars, but bought models with smaller engines.

"There is no excuse for an intermediate car today for nonpolice work," said fleet administrator Sherman Sawney, "unless there is a special application. And somebody will have to prove it to me."

Shawney said the fleet average for nonpolice cars used to be 8 to 10 miles per gallon, but should be up to 18 to 19 mpg by next year.

In Fairfax County, "we place people in the smallest vehicle that will do the job," said Fred K. Kramer, director of general services. The county went to smaller models beginning in 1974, he said, and now buys subcompacts except for police, where it has gone from full-size cars to an intermediate model.

Prince George's Doremus said that initial attempts to introduce a smaller, midsize car for police met opposition from many officers, who claimed it would be unsafe. But, he said, there have been 20 percent fewer body repairs on the smaller Chevrolet Malibus, which replaced the Chevy Nova.

"I don't know," Doremus said, "but maybe the officers are being more cautious."