New Jersey drivers could one day find themselves paying tax on every mile they drive in the Garden State.

As more and more hybrids take to the roads, transportation experts fear the state gasoline tax won't be enough over the next 25 years to cover road repairs. That has led state officials to seek alternatives.

The mileage tax, suggested in the North Jersey Transportation Planning Authority's 25-year plan released last month, could be used to replace the per-gallon tax on gasoline.

Satellites could be used to track vehicles as they crisscross the state. An in-car computer could wirelessly transmit mileage data to gas pumps that would calculate the tax.

There are significant technological and social hurdles - including privacy concerns - that would have to be solved before such a system is put in place. Still, transportation planners are intrigued.

"It's being looked at seriously because everyone recognized that this is a problem that's on the horizon," NJTPA spokesman Mark Solof said. "Within 10 to 15 years, the gas tax just isn't going to yield enough money to support infrastructure repair and maintenance."

Next year, Oregon will run a pilot program to evaluate such a system. New Jersey and other state officials nationwide plan to watch it closely.

Instead of being charged Oregon's 24-cent fuel tax, about 300 volunteers in Portland will pay a tax of 1.2 cents per mile. Satellites and electronic odometers will be used to track their mileage and apply the tax.

"Gas taxes just won't work; even if you keep increasing them, you will be forever behind," said Jim Whitty, the Oregon Department of Transportation official heading the state's experiment, the first in the nation.

"Everybody's making guesses as to when that will happen. It's not known, really. All we know is it's going to happen at some point and we have to be prepared."

Oregon officials want to be ready in a decade or so in case they have to start moving away from the gas tax, Whitty said.

Federal transportation officials - who are paying for most of the $3 million study - rely on an 18.4-cent federal gas tax to dole out highway grants.

Oregon's pilot program, scheduled to begin next spring, will outfit volunteers' vehicles with satellite tracking equipment that will count the number of miles driven within the state's boundaries.

Gas stations participating in the program will bill those volunteers under a customized system in which the mileage data are passed on to the pump wirelessly. The regular gasoline tax will be waived and motorists will be taxed on the miles driven since their last fill-up.

Whitty said the system also could tax at a higher rate for driving in congested zones or on busy highways and for driving during peak traffic times. Similar congestion pricing is now used at the Hudson River crossings, where drivers are charged more during peak periods.

And higher rates could be applied to gas guzzlers, so that motorists who drive fuel-efficient cars are not penalized under the new system.

But the prospect also has raised concerns that motorists will be surrendering personal liberties.

Whitty said the Oregon experiment will use GPS equipment that does not send out signals telling some Big Brother-like machine where it is. Rather, he said, it receives only satellite signals used to determine whether or not the car is in the state or in a certain traffic zone.

New Jersey's transportation chief isn't embracing the idea, at least not yet.

Transportation Commissioner Jack Lettiere said predictions of the gas tax's demise may be premature and that the privacy and fairness questions are significant obstacles.

"To say that we're in an emergency and to say we have to look at this right away, I don't necessarily buy that," he said. Lettiere, president of the American Association of State Highway and Transportation Officials, said he also sees no quick movement toward the kind of fuel efficiency that would make the gas tax insufficient.

He noted that fuel efficiency in the United States has declined over the past two decades from 22.1 mpg to 20.8 mpg.

"Folks are going out saying we have to do this because the motor-fuels tax is no longer viable, but that is a myth," Lettiere said. "The efficiency of the entire national fleet has gone down."

The money being raised by New Jersey's gas and diesel taxes also has continually gone up during the past several years as sport-utility vehicles proliferated and the population increased.

The state collected $566 million from motor fuels taxes - up 12 percent from 2000, according to figures from the state Treasury Department.

Even so, New Jersey transportation officials are now facing a gas tax crisis. The Transportation Trust Fund is more than $6 billion in debt, the gas tax hasn't been increased in two decades and state officials continue their longtime habit of siphoning gas tax revenues to pay for operating costs.

Unless the state taxes, now at 14.5 cents per gallon, are increased or other revenues are found, road construction projects could be sidelined next summer.

Still, many experts are looking beyond the present crisis to a day when taxing gasoline just doesn't cut it anymore, when ever-increasing oil prices and better technology prompt motorists to trade in today's gas-guzzling SUVs for tomorrow's hybrid or hydrogen-powered vehicles.

Whether it's 10 years or 30 years, transportation planners say the country's fuel consumption is going to shift, and the nation's transportation officials need to start thinking about other revenue sources.

"Something different than what we're doing now, I think, is coming," said Jeffrey Zupan, a senior fellow on transportation with the Regional Plan Association in New York, which has begun studying alternative funding sources. "At some point, we're going to wake up to the idea that we can't keep buying Hummers that get 10 miles per gallon."