I'm having the same problem in here: http://www.codeplex.com/JigLibX/WorkItem/View.aspx?WorkItemId=5109
It kinda went unresolved.
Basically the car isn't stable at high speeds. The car shudders left and right and eventually slows down. I think it has something to do with either the velocity on the tires or the tires' contact to the surface. When the tires lose and gain traction unevenly the car pulls left then right.