Fred -
We usually tip 10-15% depending on the tour guide. If there is a driver only, we usually give them a few $$$ depending on what kind of driving it is. If the tour guide is also driving, then it's toward the higher amount.
If they drive like maniacs, then they're out of luck. We have been known to tell them that I am one of "those nervous passengers" so that we have a pleasant ride and I have permission to control the car from the backseat.
