well that might be caused because the car is bigger or smaller than the one you are replacing (bigger size not MB size).
if you use automatic car installer, it changes handling by itself.
there is a realistic driving & flying mod that is pretty good.(realest vehicles handling).
if you try to change a car by a moto or viceversa it will bounce around.(doesnt mean that you've done it, is just info).
any doubts just reply.