tires are 275/60-15 drag radials
tried 15x8 stock gm steel wheel with 4 in backspace...rubbed a little

tried 15x8 weld 3.5 backspace...looked "hillbilly" and rubbed on fender bad

so ordered 15x8 weld 5.5 backspace....rubbed inside, infact would not even bolt flush to the drum, so i cut the front corners of the inner well at
the bottom, and also trimmed the bump-stop brackets, AND added a 5/16th spacer, fit perfect, no rubbing anywhere.
next set of tires will be a little smaller, but i had these already
hope this helps someone else

don't know how the hell guy's are running 6.5 backspace??

 

whos running a 6.5" BS?

I measured my wheel, a 18x9.5 with 20mm offset, its like a perfect 5.5" or 5.55 if you wanna be picky, but it fits without spacers!

(and no, not the ones in my sig, other wheels, no pics yet, its undercover )

 

I run 15x10 with 6.5 backspacing. You gotta cut and modify the inner wheel well.

15x8 with 5.5 backspacing should fit perfect.