i want to know if some Z06 17 by 8.5 w/ a +15mm offset all around will fit on a 1991 z28 i was thinking of putting some 275/40/17 on back and some 245/45/17 in front .....what y'all think

 

Sure; given that stock Z06 wheels fit, and the ones you're talking about (which are evidently NOT Z06 wheels but rather some of the cheap knock-offs that look sort of like them such as you see on eBay all the time, probably with the cheap chrome that flakes off when it gets wet) are SMALLER than the stock ones, you shouldn't have any problem at all, if you use the right spacers. Probably 2" up front and 1¾" in back.

A set of ACTUAL stock wheels, such as I have on my car, are 17" front and 18" rear; IIRC 275/40-17 in front and 315/35-18 in the rear. I could be wrong about the exact tire sizes though, but you can just look up stock 97 Vette tires on TireRack and see how much smaller than stock those supposed "replicas" are. The real ones however require 2" spacers in the rear as well, because they'll rub on the inside of the fender well at the front bottom of the wheel wells, right where the sheet metal covers up the control arm mounts.