How fat can the rear tires be on a 1996 z28. With 18" Rims.

Will 11" work?

 

18" wheels means nothing other than how tall the wheel is, this says nothing about how wide the wheel is from lip to lip, or bead to bead. If it is 9.5" from bead to bead, go with a 275. If you wanted yes you could run a 285.

The Z06 runs a 285 on a 10.5" wide wheel, and Porsche pushes it further with a 275 on an 11" wide wheel (the minimum for a 275).

 

Actually 295/R18 tires can fit....315's are usually for 17's

 

I know people have have even run 315 on 17x9" wide rims...I wouldnt recommend it though.