Ok, for factory C5 Z06 wheels 17F/18R, I know I won't have any problems with the fronts, but the 18's on the back, the offset is...56? Will the GM Factory 18 Z06 wheel and stock sized tires clear the rear fenders, or will some hammer work be required? Thanks!

 

According to data from an old issue of VETTE magazine, the C5 Z06 factory wheels are:

17x9.5" 54mm offset
18x10.5" 58mm offset

The 17" wheel will fit front or back with no problems. But the 18" wheel can be a problem, since the backspace is about 8.0". That puts a wide tire into the inner fender liner.

I suggest you find others here who actually have the factory Z06 wheels and verify with them any problems they had with the 18x10.5" factory wheels. I'm just relying on data from a magazine, and converting it to backspace.