Wheel back space question…..

My car is a '73 Scamp. Has 7 1/4 rear end now, just picked up an 8 3/4 last night. I want to run 15x8 rallye wheels w/245/60/15 for now. I have a 3' re-location kit also, what back space is needed for 15x8 rim? I also plan to run 15x10 rallye wheels later on w/a 275/60/16 or 275/50/15.
Any help greatly appreciated.
I've been messuring twice, I have yet to make or start any cuts to car/wheel house or anywhere till I have more info.
Thanks in advance......

Mark
Author: admin