Zipcar, a pay-by-the-hour car rental service, is interested in expanding its Ann Arbor fleet later this year.

In November, the Cambridge, Mass., company plans to add to its fleet of 8 vehicles serving downtown, said Nancy Shore, executive director of the getDowntown program, which promotes public transportation initiatives in downtown Ann Arbor.

"The last time I talked to them, they wanted to add more cars, because the program is so successful," Shore said.

Zipcar spokeswoman Colleen McCormick said the company does not provide information about expansion plans.

Zipcar started with four vehicles in downtown Ann Arbor in 2009 with a financial guarantee from the city's Downtown Development Authority and today maintains 8 cars downtown and 20 vehicles on campus.

The DDA no longer pays a vehicle charge, which was reduced over time as the program began to pay for itself through hourly usage.

While Zipcar won't provide specific membership numbers in each of the cities and campuses where it operates, McCormick said membership grew in Ann Arbor by 56 percent from April of last year to April of this year. During the same time, membership grew 35 percent from the University of Michigan.

Zipcar has approximately 577,000 members in the U.S., Canada and the United Kingdom who pay an annual rate of about $50 and an hourly usage rate (in the U.S., $8 on weekdays and $9 on weekends) that covers the cost of gas.

All drivers need to do is reserve vehicles in advance either by phone or online and park the car in the same spot they found it. A keycard turns on the vehicle, which will be parked in a Zipcar-dedicated spot. The cars, which come in various makes and sizes, are also serviced by the company.

"I get overwhelmingly positive feedback from employees who work downtown and use the cars to run errands or go to a meeting," Shore said. "I've heard from residents that it allows them not to have a vehicle or to have only one vehicle. It's appealing to a lot of people because Ann Arbor is walkable and accessible to public transportation."

Shore said Zipcar has asked her not to share usage data.

Zipcar had an initial public offering in April that raised $174 million and drove up its share price from $10 to $28 the first day of trading. As of May 12, its stock (Nasdaq: ZIP) traded at $27.65 a share.

The company's earnings report shows that in the first quarter of this year, revenue increased 48 percent to $49.1 million, compared to $33.2 million during the same period last year. Usage revenue accounted for $41.9 million of the revenues.

Zipcar maintains a global fleet of 8,000 vehicles.

The company also now has a presence on the Michigan State University campus.

Julie Edgar is a freelance writer for AnnArbor.com