Originally posted by 66chevyIISS
but a exmaple:
for drag racing: I am going to run a 15" wheel over a 18" wheel in the rear of the car. More sidewall, better launch traction.
For road racing/handling: I am going to run a 18" wheel over a 15" wheel 
less sidewall, better grip for corners.
Some vehicles aren't that fortunate.
I couldn't fit a 15" rim over my discs/calipers if my vehicles life depended on it. : ( The 16's I have on there barely fit.
But thats what I would do if I could.
I believe the 300ZX has slightly bigger discs than my car does.