GM announced a new service it is providing for customers buying new Chevrolet, Buick, Cadillac and GMC vehicles. The service, which is an arrangement between GM, OnStar, and RelayRides, a Boston-based car-sharing company, gives owners the chance to make "hundreds of dollars a month."

Whether or not allowing your vehicle to be rented by strangers is a smart choice is something consumers will have to decide for themselves, but for now, the folks at Hertz, Enterprise and Avis can't be too happy... The new GM deal is estimated to add 6 million vehicles to the pool of available rental vehicles.

And while 'car-sharing' via options like ZipCars is gaining interest in metro areas like New York and Boston Car-sharing; ZipCars this new arrangement lets consumers interested in a vehicle have a chance to "rent" it cheaply and really see if it's the right vehicle for them.

In his report for HybridCars.com, Huw Evans, said that for consumers interested in, but not yet willing to purchase an electric vehicle, the GM/OnStar/RelayRides arrangement presents a good opportunity to sample cars like the Chevy Volt without making a significant upfront financial commitment.

According to automotive expert Justin Hyde of Motoramic, RelayRides was launched in 2010. It lets car owners set their own hourly rates --typically $5 to $12 per hour-- and takes 35 percent of the revenues as its fee. Potential renters surf the company website and pick cars by profile. The rental fee includes gasand 20 miles of driving along with extra insurance and roadside assistance.

RelayRides says some owners can make up to $650 a month from the service. Before the GM/OnStar agreement with RelayRides, enterprising car owners had to meet renters in person to drop off and pick up keys, or install a special card reader on the outside of their vehicle that allows renters access at specific times.

Now, by using OnStar's ability to lock and unlock vehicles remotely, renters are able to get into an OnStar-equipped car via a smart phone or replying to a text message; and that one of the reasons GM invested in RelayRides.

Not surprisingly, there are a few glitches that may need to be worked and insurance companies are raising objections; warning people who rent their vehicles out that such usage violates their personal insurance policy and could lead to higher rates, or being dropped from coverage.

But for now, some 6 million GM vehicle owners with OnStar can rent them out by the hour... if they want to assume that risk.