The 36s are wider and taller, but have a lower weight rating. Thats about it that I know of. I have 37s I bought used. I wanted the 36s. Sitting next to my friend who has 36s, they look the same to me as far as one being more square or round than the other.
~T.J.
EDIT: The look might have something to do with the wheels theyre on too. Mine are on some 15x8s or some narrow wheel like that, so that might make them balloon up more. If they were on a 15x10 or 15x12 wheel, they would probably look more square. Then again, the 36s may do the same thing :eeek: