moving entities can't use the poly collision, only static models, like your curbs and road. so your cannot set my.polygon=on for any moving entity that has to have collision detection.

your car model using c_move and c_rotate uses an elipsoide collision hull
(The older ent_move used a box hull but with no rotation collision detection)
if you scale your car it may need to have its collision hull reset
( c_setminmax() )