Well, I have never done auto races, but for any action shot, you need fast shutter and bright light. I would recommend aperture at about 4.0 and shutter as fast as you can, but at least 1/1200. I would also recommend using the HS multishot mode. Focus you will want to play with, I have had good luck with some things using single mode, focusing manually at a given point, and then photographing in HS as the subject approaches goes by that focal point. It has worked well for me. Play with it, try it out, and KEEP THAT MANUAL WITH YOU. There are a lot of secrets contained in those pages... TRIPOD HELPS ALOT.
I will be boating this weekend, so I will try and get some shots of other boats zooming by to show some examples.
Good Luck,