Whether you own your own or are just renting one for the day, hybrid cars are all the rage this summer.

Ever-rising gas prices mean that more people are choosing to rent rather than maintain vehicles of their own. To keep up with this increase in membership, car-sharing companies are looking to add extra vehicles to their systems - particularly those that are environmentally friendly.

According to PhillyCarShare Deputy Executive Director Clayton Lane, when gas prices topped $4 per gallon in the Philadelphia area during the first week of June, PhillyCarShare saw an immediate spike in applications.

"We see a close relationship between high gas prices and membership," said Lane. "Right now we'll have about 3,000 people signing up in the month of June . 40 percent higher than last year at this time."

Just a few weeks ago, PhillyCarShare added their 50,000th new member, making it the largest regional car-sharing industry in the world, Lane said.

To accommodate increased demand, PhillyCarShare recently expanded its service to include 40 cars on Penn's campus and is looking to add more hybrid vehicles to their options.

For now, "over half of our cars are hybrids," said Lane. "That's the highest in the whole car-sharing industry."

And PhillyCarshare isn't the only car-sharing company that has been affected by the skyrocketing gas prices.

According to Philadelphia's Zipcar general manager Jeremy Nelson, 45 percent of their members have named gas prices as a reason to join Zipcar.

Rather than expand the number of vehicles to allow for the increase in customers, Zipcar recently discounted many hybrid cars to just $6 an hour.

"Hybrid cars have always been a part of our system," said Nelson. "We bumped the price to encourage more people to buy them."

As fuel costs continue to rise, so do the companies' expenses. However, according to representatives from both PhillyCarShare and Zipcar, the membership revenue far outweighs the expenditure on gas.

And Zipcar offers an additional benefit to customers. According to Zipcar spokeswoman Maria Martinez, because gas prices are a small percentage of Zipcar's overall operating costs, the company does not frequently have to adjust pricing due to rising fuel costs.

As for college students, the cost of membership is definitely worth it.

"I use [PhillyCarShare] all the time," said College junior Lucy Voorhees, who added that the company offers her and her friends "the convenience of having a car but not having to own a car in the city."

However, PhillyCarShare's ultimate goal is just to get more cars off the road.

"Our bottom line is social," said Lane, adding that the company is trying to get people to become less dependent on automobiles. "When people join [PhillyCarShare], they are changing their lifestyles."