You have to be careful about the rolling circumference of the tires, larger or smaller and they will be working against the rears and you will wind up with some very costly repairs. Im sure someone can give you a better explanation then that
Todd,
Skeets hit the nail on the head - perfect explanation.
imagine this - you walk, both legs the same, then for some wild reason one leg a bit shorter than the other! The impact on all the parts will hurt, hips, knees etc.
You have to keep the turning ratio front to rear reaaallll close to the original ratio that came from the factory - otherwise a bind will occur and as Skeet said, you wallet now has a hole in it .....rather large hole!