Gary,
Like Dale said, bigger usually fits better on the back, and besides that, it doesnt have to turn. Another reason is so you can get the best traction in the back...Small in the front, so there is less rolling resistance(Im talking about those tiny pizza cutting kind). But that would be pretty much for drag racing only. Mine have bigger ones in the back, and smaller in the front...but they are the biggest you can fit in the front....I like to be able to turn, and have some tire there for a smoother ride.
-Tim-
1966 Chevelle
