It’s a rule of life most human beings learn before they obtain a driver’s license: cars cost money. Big money.

First, you have to have enough to buy one. Then you need enough for gas, insurance, general maintenance, parking, etc. Let’s not forget that as soon as a new vehicle is driven off the lot, its value depreciates.

But now, because of a partnership between peer-to-peer car-sharing service RelayRides and automaker General Motors (NYSE:GM), there’s an new way for some vehicle-owners to make extra dough — by renting out their wheels. Peer-to-peer car-sharing isn’t a brand-new concept, but GM and RelayRides have made the process easier for their customers through the partnership.

Here’s the skinny on the new service.

• GM’s OnStar subscribers can now use their smartphones to access RelayRides’ marketplace to rent out their vehicles.
• The gas is provided by the vehicle’s owner, but renters are required to replace the fuel.
• The rental rate is determined by the owner. The minimum fee is $5 per hour. The owner cannot charge more than $50 an hour.
• For owners, RelayRides provides a $1 million liability insurance policy and strict renter screening.
• For renters, RelayRides provides insurance coverage and 24/7 roadside assistance and support.
• Renters can use their smartphone in conjunction with OnStar to unlock the door of the rented vehicle (keys will be inside).

The concept of peer-to-peer car rentals has grown increasingly popular over the past few years, especially among those living in large metropolitan areas who don’t want the hassles associated with car ownership.