I have a car; many of my friends do not. I am happy to share with them, and I've never had any rules about using it, just an expectation that people would buy gas, or at least buy me a drink later that night. But recently I let a friend use my car while I was out of town, and he put 800 miles on it. Did he take advantage of my generosity? I have a car payment and pretty high insurance; should I ask friends for reimbursement when they use my car?

A lot of car owners fear becoming a one-person Zipcar operation, less because of daily wear on the vehicle than because of the risk of a friend crashing it. But sharing a car is commendable. It's good news for both the community and the environment — and there are ways to avoid getting burned by your joy-riding buddies.

First, call your insurance company to check the details of your policy. If you have full coverage, it will usually cover another driver who has permission to drive your car. The risk, however, is that an accident with anyone at the wheel can drive up your monthly premium. Depending on the policy, your premium may rise even if it's not the driver's fault — if, say, your highly responsible friend is rear-ended at a red light. Then there's the unlimited mileage issue. It seems like one thing to let your friends use the car to schlep to the grocery store or to get out of town for a day hike, and another thing to let them take it on a serious road trip. Charging them doesn't seem like a great option — it's awkward, and it doesn't really solve the issue — but you should at least get to say yes or no to the trip itself.

So, second, set some ground rules. Drop a note to the folks who use your car, along the following lines: "Dear friends, you are all marvelous drivers and wonderful people, and you remain welcome to use my car. But since I am financially liable for your on-road adventures, please: 1) Buy your own gas. 2) If you get into an accident while using the car, help defray any change in my premium. 3) If you want to use the car for more than a day trip, run it by me first. Okay? Thanks!" Your friends are still getting a terrific deal, and I suspect they'll have no problem with these conditions. If they do, well, there's always Enterprise.

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