Patrick Conner's Nissan Leaf electric car sports a custom plate as he shows how to plug it in at a charging station Tuesday at the public library in Hillsboro, Ore.

PORTLAND, Ore. — Oregon is about to embark on a first-in-the-nation program that aims to charge car owners not for the fuel they use, but for the miles they drive.

The program is meant to help the state raise more revenue to pay for road and bridge projects at a time when money generated from gasoline taxes are declining across the country, in part, because of greater fuel efficiency and the increasing popularity of fuel-efficient, hybrid and electric cars.

Starting July 1, up to 5,000 volunteers in Oregon can sign up to drive with devices that collect data on how much they have driven and where. The volunteers will agree to pay 1.5 cents for each mile traveled on public roads within Oregon, instead of the tax now added when filling up at the pump.

Some electric and hybrid car owners, however, say the new tax would be unfair to them and would discourage purchasing of green vehicles.

“This program targets hybrid and electric vehicles, so it’s discriminatory,” said Patrick Connor, a Beaverton resident who has been driving an electric car since 2007.

Officials say it is only fair for owners of green vehicles to be charged for maintaining roads, just as owners of gas-powered vehicles do.

“We know in the future, our ability to pay for maintenance and repair ... will be severely impacted if we continue to rely on the gas tax,” said Shelley Snow with the Oregon Department of Transportation.

Other states are also looking at pay-per-mile as an alternative to dwindling fuel tax revenues.

Last year, California created a committee to study alternatives to the gas tax and design a pilot program; Washington state set money aside to further develop a similar program; and an Indiana bill directs the state to study alternatives and a test project.

While growing in popularity, electric vehicles and hybrids are still in the minority on American roads, even in a state as green-minded as Oregon. Of 3.3 million passenger cars registered in Oregon at the end of 2014, about 68,000 were hybrid, 3,500 electric and 620 plug-in hybrid. A decade ago, only 8,000 hybrids were registered.