So in the absence of getting custom backspacing, it would be a fairly safe bet a 255/60/15 on the 15x8 with 4.5 backspacing would fit just fine hey (on paper anyway)?
“On paper” that combination will fit, but it may still be tight. Clearance to the springs would be about 3/8” by the math using my Duster when it had a 7.25” and I test fit a set of 275/40/17’s as the comparison.
As always, I’d say measure your car. The 255’s should work, but you don’t have much leeway to work with if the tolerances on your car don’t match mine. And with the larger sidewalls it’s possible you might still get some rubbing under certain conditions if you cut it too close.
