aahhh... thats the part i was missing.
(i was assuming same side wall size... which, duh, doesn't happen) even i was looking out the window at my car, and looking at ion's on the main page and scratching my head, at how stupid of a question that was.
hey! medical diagnosis is easy.. automotive anything is tricky. there is no rhyme or reason, engineers just do, and let people figure it out later.