Until this year, the Arlington personal property taxes on Frank Roth's 1974 tan Mercedes-Benz seemed like a pretty good deal.

"I was bragging a while ago about my car, saying that by keeping this old car, I'm paying only $90 a year in taxes," said Roth. The latest personal property tax statement from the county, though, put a brake on Roth's boasting.

Under a new, uniform method of valuing older cars, mandated by the Virginia General Assembly in March, the assessed value of Roth's Mercedes-Benz lurched from $1,980 to $4,750. The tax statement, which arrived last week, said he owes the county $221.

"The car is now a year older, but I have to pay more than twice as much in taxes," Roth complained.

Before this year, Arlington assessed cars more than seven years old by approximation, taking 70 percent of the previous year's value, down to a $200 minimum, explained Deputy Commissioner of the Revenue Ruth Kanter. The personal property tax rate is $4.65 per $100 of assessed value.

More recent models were assessed according to the National Automobile Dealers Association Official Used Car Guide, a tiny paperback bible of loan values, retail values and trade-in values for everything from Plymouths to Porsches.

Now, vintage Camaros, Cadillacs and Chevys are valued by the book -- a separate edition of the NADA guide for models from 1968-1977.

For Eugene Hagar's 1969 burgundy Pontiac Firebird, that means a jump in taxes of about 1,000 percent. His car, valued at the $200 minimum last year, now is assessed at $2,100. "You can imagine my shortness of breath upon getting the notice," Hagar said. "I thought it was a rather strange way of increasing my taxes."

The new rule, which mandates that cars and trucks of less than two tons be valued by means of a "recognized pricing guide," aimed to bring uniformity to older car assessments that varied widely across the state, Kanter said.

"It's a rip-off," said Roth, calling the change "legislative hocus-pocus. Part of the reason I bought this car was that I knew it wasn't valued highly."

Alexandria has been using the official guide for older cars for several years; until this year Fairfax County used a market survey of the cars' average trade-in values as a basis for assessments.

Kanter estimated that several thousand of the 120,000 cars Arlington taxed this year will be affected by the new assessments; she could not say how much revenue the county would receive as a result.

"For some unusually interesting cars," particularly hard-to-find convertibles, "it can be a considerable tax," she said.

This week, Kanter paged through a thick sheaf of complaint slips from the hundreds of irate owners of vintage cars who have phoned her office since the assessments were mailed last week, and smiled as she explained the new system to the latest caller.

"Essentially, we've been undervaluing your car," she told the owner of a '69 Camaro.

The owner wanted to see it in writing. "I'll send you a copy of the book," Kanter assured the caller. "Look at it this way: You've gotten a good deal in the past."