I think it looks pretty good, too, but here's a workaround:
Make wheels invisible, create visible but passable copies. Now set the position of the visible tires as follows:

vec_set(invisible.pan,visible.pan);
invisible.x = visible.x;
invisible.y = visible.y;
if(invisible.z > visible.z) invisible.z = visible.z; // high to low surface works smoothly
else invisible.z += (visible.z-invisible.z)*time_step;