With vehicles using less gas, but vehicle miles increasing, Virginia transportation officials have begun thinking about linking the highway gasoline tax to the miles that vehicles drive on the state's roads.

Doing that, however, would not be a painless fix for the state's highway woes.

"It would mean that the gas tax itself would go up," Ralph M. Davis, state deputy secretary of transportation, said in an interview.

The buying power of Virginia's fuel-tax revenue lags behind the growth in the amount of driving being done on the state's roads. But the number of miles that vehicles drive also drives the need to construct and repair roads.

Fuel-tax revenues "are basically not going to keep pace with the cost of building roads, are not going to keep pace with the cost of maintaining roads," Davis said.

Virginia's motor-fuels tax brought in $851.2 million last year. Since Virginia's gasoline tax of 17.5 cents per gallon was adopted in 1986, inflation has cut its purchasing power almost in half.

And just in the period from 2000 to 2007, the most recent year for which figures are available, vehicle-miles traveled in Virginia grew 13 percent, from 199.1 million to 224.9 million, according to the Virginia Department of Transportation.

"We definitely need to be thinking about alternatives to the gas tax over the next decade or so," said Commonwealth Transportation Board Member Gerald P. McCarthy of Richmond, "whether it's [vehicle-miles traveled] or some other [measure] to provide sustainable revenues for transportation."

The Transportation Board has been slashing transportation spending as the recession has eaten into its highway and transit revenues.

Vehicle-miles traveled -- the number of miles driven by cars and trucks -- is a key measure of highway use. Until recently, gas and diesel consumption has been a rough-and-ready proxy for miles driven.

But as vehicles -- such as the popular Toyota Prius hybrid gas-electric car -- become more fuel efficient, gas-tax revenues will dwindle in proportion to vehicle-miles traveled. And cars and trucks running purely on electricity, natural gas or hydrogen pay no motor-fuels tax at all for the miles they drive on public roads.

National discussion of using a mileage tax to replace the cents-per-gallon fuels tax has focused on using the Global Positioning System to record a vehicle's mileage and report it to the government.

Transitioning to a GPS-based tax on vehicle miles traveled would be technically and politically difficult, officials agree, and making such a shift would take upwards of two decades.

However, Virginia could create a sort of mileage tax by annually adjusting the motor-fuel tax rate by the percentage change in statewide vehicle-miles traveled, Davis said, and add a factor to protect the revenue source from inflation.

"It's a concept, an option for our consideration," he said, and not a proposal for action by the administration of Gov. Timothy M. Kaine.

In one example of how the tax might work, Davis calculated that the pseudo-mileage tax would add about 2 cents a gallon to the price of gasoline and raise about $102 million a year for state transportation needs. His example used 2000 as the base year and the statewide vehicle-miles traveled and the inflation rate from 2007.

The state's truckers are not enthusiastic about mileage-based taxation.

"Our industry's position is that the diesel tax is the best and most efficient way of collecting highway-user dollars from us," said Dale Bennett, executive vice president of the Virginia Trucking Association, which has 550 member companies.