Avis is paying a 49 percent premium on Zipcar's stock price, and of course, the company isn't only buying Zipcar's reputation. Avis said it expects $50 to $70 million in annual cost savings from the deal as it leverages its fleet across the two companies, and that access to Avis's vehicles will let Zipcar grow in new markets. (For a sense of scale, Zipcar says it owns more than 10,000 vehicles; Avis claims 10,000 rental locations.) Meanwhile, the rental car giant also expects to benefit from Zipcar's mobile technology.

Per Avis CEO Ronald L. Nelson:

We see car sharing as highly complementary to traditional car rental, with rapid growth potential and representing a scalable opportunity for us as a combined company. We expect to apply Avis Budget's experience and efficiencies of fleet management with Zipcar's proven, customer-friendly technology to accelerate the growth of the Zipcar brand and to provide more options for Zipsters in more places. We also expect to leverage Zipcar's technology to expand mobility solutions under the Avis and Budget brands.

That all sounds reasonable enough, and Avis isn't the first rental giant to make a play for car-sharing. Avis's larger rivals, Enterprise and Hertz, already operate car-share businesses, and Enterprise acquired Mint Cars On-Demand and PhillyCarShare last year.

Still, we can't help but worry. Please don't screw up Zipcar, Avis.

Oh, for an old-media expense account!

Well, here's one way to cover the exorbitant cost of a parking space in New York City: peer-to-peer car-sharing marketplace RelayRides just announced a big partnership with General Motors and OnStar, meant to make it easier for drivers to rent out their idle vehicles, Airbnb-style.

That is, if you can convince yourself to trust a complete stranger with your car.

RelayRides announced today that OnStar subscribers can now leverage the system to make renting out their cars simpler. Renters will be able to unlock OnStar-equipped cars using only their mobile phones, a.k.a. the one device most of us are practically guaranteed never to lose.

The company launched in Boston just over two years ago, then expanded to San Francisco in late 2010. A nationwide launch came in March, though up until now renters have typically had to pick up the keys in person.

Besides simplifying the car exchange process, it sounds like this partnership is another way for RelayRides to confront that whole "giving up your car" issue. From a statement released today:

“Using the OnStar API to access GM vehicles empowers RelayRides to make car sharing even safer and more convenient,” said Shelby Clark, RelayRides founder and Chief Community Officer. “The sheer number of vehicles eligible for the program allows us to greatly expand across the U.S. and introduce the economic, environmental and community benefits of car sharing to regions that car sharing services have previously been unable to serve.”

The company also promises a $1 million insurance policy for owners and $300,000 for renters. That would cover a lot of cab-inflicted damage, we suppose.

In fact, it seems like the demand side might be more of a problem. This Betabeat reporter, for one, is awfully attached to her Zipcar subscription, and that particular Boston-born company has clearly invested a lot of time, money and effort into ensuring that wherever a New Yorker wants a car, there'll be one within a reasonable distance. They even cover the gas, which at least removes the price-point pain. (As for finding a gas station, well, good luck and godspeed to you.) 

Then again, from a business perspective, we would imagine it's rather simpler to be the marketplace, rather than maintaining a Zipcar-scale network of vehicles.