I was having a problem using Newton with cars, I was not able to do the cars collide. They could collide with the track geometry, but not between them.

I discovered that there is an error in the NewtonVehicle and NewtonScript2 ( If you using the Newton Car Demo ), overlapping the value of the Skill8, that is used for CarTopSpeed AND Collision_GroupId at the same time.

If you change the Collision_GroupId to, for example, Skill11 in all your scripts, everything will work fine and the cars will crash very very solid

Hope I made a good english...

And hope this can be usefull to someone else.