PORTLAND, Ore. — Oregon is about to test a first-in-the-nation program to charge car owners not for fuel used, but for miles driven. The goal is more revenue to pay for road and bridge projects as gasoline tax revenues are declining across the country because of greater fuel efficiency and the 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 mileage data. They will agree to pay 1.5 cents for each mile traveled on public roads in Oregon, instead of the regular gas-pump tax. Some electric and hybrid car owners, however, say the new tax would be unfair to them and would discourage purchases of green vehicles. State officials say it is only fair for owners of green vehicles to be charged for maintaining roads. ‘‘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 of the Oregon Department of Transportation. California is studying alternatives to the gas tax, Washington state has set money aside to develop a program similar to Oregon’s, and an Indiana bill would direct that state to study gas tax alternatives and a test project. — ASSOCIATED PRESS