Marco Monster has a nice tutorial article on Car Physics (mirrored here) that’s worth reading if you are thinking about writing your own version of Grand Tourismo. There is also a ton of good links at the motorsport sim Wiki.
Addendum: Brian Beckman’s Physics of Racing series can be found here as a set of 30 PDF files. Those of you with the desire to not click 30 times can simply type:
wget -l 1 -A.pdf -nd -np -r -erobots=off http://phors.locost7.info/contents.htm