How annoying! Sorry to post in such an old thread but I'm having the exact same problem. There doesn't seem to be a lot of help around for this thing. I've tried to do my own code for this, and it seems totally logical but it's kind of faulty for some reason. Here is the code I used. It won't apply to your game obviously but you can change it and use it. It works out the 'tilt' for more than one car though:
rem Calculate degree of tilting from corner heights
If object position y(Car)=Get ground height(1,object position x(Car),object position z(Car))
FLYPos#=get ground height(1,(object position x(Car))-2,(Object position z(car))+3.5)
FRYPos#=get ground height(1,(object position x(Car))+2,(Object position z(car))+3.5)
BLYPos#=get ground height(1,(object position x(Car))-2,(Object position z(car))-3.5)
BRYPos#=get ground height(1,(object position x(Car))+2,(Object position z(car))-3.5)
ZTilt#=((FRYPos#-FLYPos#)+(BRYPos#-BLYPos#))*6
XTilt#=((BLYPos#-FLYPos#)+(BRYPos#-FRYPos#))*6
Rotate object car,XTilt#,object angle y(car),ZTilt#
Endif
Next car
This is all I've got so I need some more help on the subject.
Noah