The principle behind racing games is to use Waypoints. These are hidden objects that your vehicles must collide with in turn. You put them on corners and other significant parts of the track.
If you are asking questions about moving cars and collision, then I think you need to take a step back, and start smaller. Work out how to do the individual components of your game before putting them together.
The Assets and Templates Tutorial I wrote in Issue 29 of the newsletter (
http://www.thegamecreators.com/data/newsletter/newsletter_issue_29.html) uses the example of vehicles travelling around a track, which you can download with source. It's extremely simple, and doesn't use collision between vehicles, but it will introduce you to a few of the concepts you need to get your head around.
BatVink
