When Jason Maupin, 24, returned to Pittsburgh last year after a student externship in Hershey, Dauphin County, he decided to make a lifestyle change.

He sold his car.

"I realized the expense it would be to have a car," he said. And he also saw "an opportunity to be a little more environmentally conscious and healthy."

Living at Allegheny Center meant that he could walk to classes at the main campus of Community College of Allegheny County, and to his job at a Downtown coffee shop.

For those occasions when driving was the only feasible option, he joined Flexcar, the Seattle-based car-sharing service that began operations in Pittsburgh in May, and merged with Cambridge, Mass.-based Zipcar in November.

By December, he was encouraging his friends to join. In January, he stopped, having second thoughts about his own membership.

The reason? A new county tax that went into effect Jan. 1, alongside the drink tax. The car rental excise tax, passed by County Council in an 11-4 vote, is $2 per day, "or any part of a day," for every car rented in Allegheny County.

Car rental excise taxes are nothing new. Customers have paid an additional $2 a day for rentals in Pennsylvania since 1991, for the state Public Transportation Assistance Fund.

But the clause, "or any part of a day," is a twist that can make the tax especially onerous for Zipcar customers, who may rent a car for as little as half an hour, or may even make multiple reservations in a single day. A notice posted on Flexcar's Web site (which members can still use to make reservations during the transition to Zipcar), reads, "The tax will be $2 per reservation (plus $2 for each additional or partial day, if your reservation is longer than 24 hours)."

"My guess is that this is unique" among the 100-plus car rental excise taxes across the nation, said Caleb Miller, spokesman for the National Business Travel Association. His group is part of the Coalition Against Discriminatory Car Rental Excise Taxes, which was formed in 2006 to raise awareness of an "explosion" in such taxes since they first appeared in 1976. Other members of the coalition include the American Car Rental Association and the National Consumers League.

"It seems poorly designed," Patrick Fleenor, chief economist at the Washington, D.C.-based Tax Foundation, said of the tax. "Every time you rent the car you pay the tax."

He suggested an alternative method of administering the tax for car-sharing customers.

"Divide the daily tax by 24, and then prorate it," he said. "That would seem to be more efficient."

Perhaps, but Zipcar spokesman John Williams said efficiency was not the entire point.

"Fundamentally, we believe that car sharing is different from car rental," he said. "The question isn't so much about percentage rates as it is a question about smart policy."

Mr. Wiliams said that after people join car-sharing programs, "there is a behavior shift" -- they drive 40 percent fewer miles, and eventually many of them, like Mr. Maupin, sell their cars.

Government should encourage those types of changes, he said.

"There's lots of examples of tax policy or incentives being used to help people make smart environmental choices."

He said that legislation in at least two cities, Portland, Ore., and Chicago, has defined car sharing as different from car rental, a distinction that exempts car-sharing services from car rental excise taxes. Meanwhile, Boston recognizes the distinction by charging Zipcar members a flat $10 annual convention center tax rather than the $10-per-use tax paid on car rentals, he said.

And while Mr. Williams said the issue is one of policy rather than percentage rates, he also pointed out that the county's new tax adds 20 percent to the $10 average Zipcar bill.

Company officials have been in conversations with Allegheny County Chief Executive Dan Onorato in hopes of persuading him that the county tax code should encourage car-sharing. While declining to discuss specifics, Mr. Williams said those conversations have been "encouraging."

County spokesman Kevin Evanto said that in response to Zipcar's concerns, solicitors for both Mr. Onorato's office and the county treasurer's office were looking at ordinances in other metropolitan areas to see how they have addressed the application of car rental excise taxes to car-sharing services. He said no timeline has been set for this "fact-finding and comparison stage" of reconsidering the tax.

The car rental excise tax, along with the drink tax, was presented as a means of funding the Port Authority. Jake Haulk, president of the Allegheny Institute, a free-market think tank, said that was not accurate. Mr. Haulk said that last summer's passage of Act 44 met the Port Authority's needs, and that the new taxes were actually put in place to fill a $30 million hole in the county budget, separate from the transit agency.

He added that the tax did not generate as much controversy as the drink tax because "there are so many more people affected by that," and because many people assume that a car rental tax will hit visitors rather than locals.

But that assumption is incorrect, he said: "Somewhere between 40 and 50 percent of the tax will be collected, probably, on local residents or businesses."

The economic law of demand says that as the price of something -- including the taxation of it -- increases, the demand for it will decrease. That has proven true in Mr. Maupin's case. After renting from Flexcar/Zipcar four times in December, he did so only once in January, and said, "It would have to be a pretty important errand" to make him use the service anytime soon.

But even more, "I might buy a car."