How well can a motorcycle mechanic work on cars?

With the exception of specific information about a particular car, the principles are exactly the same. Sometimes a vehicle will have some common problems that a mechanic wouldn't know about if he doesn't work on them regularly, but that shouldn't really matter much for the more typical problems. If you have a friend or discount mechanic who can help you with your car, don't be concerned. Being able to figure out the problems with your car isn't that much different than figuring out the problems with a motorcycle.