Have a pair of new, never out of the box ZR1s. These are 50mm offset which is about 8" backspace (ideal for C5 rears). These will make perfect track wheels and if you like I have a pair of 4/32" (new 6/32") BFG DRs -315/35/17 which can go with them.
I have emailed to you for details. We understand these 11" ers will fit the rear of a C4 without fender Mods, but want to be ABSOLUTELY CERTAIN of that before committing to them.
they are the same size as stock grandsport wheels except with a different back space... can't remember if there is interference or not using ZR-1 wheels....