RALEIGH --
North Carolinians are driving more miles every year, but they're buying less gas.

Better fuel economy sounds great for the pocketbook and good for the planet, but it spells trouble for the state's reliance on gas-tax money to pay for transit and highways.

The federal Highway Trust Fund is expected to run out of money for road projects by 2009. So, as part of a $16.5 million nationwide study over the next two years, 450 drivers in the Raleigh-Durham-Chapel Hill area will help road-test a new way to pay for transportation -- by the mile, not the gallon.

"In the old days, when cars got 13 or 14 miles to a gallon, we were pretty flush with cash," said David Forkenbrock of the University of Iowa. "But we're already seeing major drops in the revenues coming in. We know it's going to get worse."

Forkenbrock will oversee the Road User Charge Study in North Carolina and five other states.

Replacing the fuel tax with a mileage fee would be a long-range idea -- and possibly a long shot.

Lew Rentel of Morrisville is one reason road money is running low -- he drives a gas-sipping Toyota Prius.

Rentel, 69, used to drive a hulking Lincoln Aviator that burned a gallon of gas every 13 miles. With 48.6 cents in state and federal taxes per gallon, he was paying the government 3.7 cents for every mile he drove.

But he ditched the luxury SUV to help fight global warming and cut our need for foreign oil, he said. Now with a thrifty hybrid that gets 44 miles per gallon, Rentel pays barely a penny per mile in tax.

"Something's going to have to be done," said Rentel, a retired UPS executive. "You're either going to tax by the mile, or you're going to tax some other way."

The federal Highway Trust Fund relies mostly on gas-tax money to pay for state road construction. The fund is expected to drop from an $8.1 billion surplus this year to a $1.7 billion deficit by 2009.

A study commission is looking for new ways to pay the bills. Congress has held the federal gas tax at 18.4 cents per gallon since 1993. The Bush administration wants more toll roads, private investment and local taxes to pay the difference.

North Carolina collects 30.2 cents per gallon of gas. N.C. gas sales have dipped since 2004. The state gas tax has jumped 5.6 cents per gallon since then, pushing total gas-tax collections up slightly to a peak of $1.23 billion in 2006. If gas sales keep falling without more gas-tax rate hikes, tax collections will decline, too.

Traffic counts are climbing twice as fast as the state population, and road construction costs are also rising. The $100 worth of asphalt, steel and concrete that N.C. road builders bought in 2002 costs more than $175 today.

The Iowa researchers will outfit volunteers' cars with computers and satellite gear to record where and how far they drive. Their mileage fees will be compared with the per-gallon taxes motorists pay now.

Congress is considering boosting new-car fuel efficiency standards by about 40 percent, to an average of 35 mpg, by 2020. By then, some Americans will be driving cars that use no gas or diesel fuel -- and pay no fuel taxes.

"As people drive around in cars that don't burn gas, we're going to have to find a way to get those folks to contribute their share to road construction and maintenance, as well," said Joseph Hummer, an N.C. State University engineering professor. "A tax that's pinned to just one fuel source makes you vulnerable."