I have 37x12.50r15 on 15x12 with 4in backspacing. Running 1.5 spacers on the front straight axle. Have a wider rear leaf sprung axle. When I flex out the rear inside on the tires rubs the paint off on the frame. I just ordered 38.5x14.50 for 15" rims. I assume I'd go with 4in back spacing on the new rims, could I potentially have more problems with rubbing? 3.5in backspacing doesn't seem like much of a difference from 4". Not sure which i should choose for my application.

 

those are both heavy wide tires.
i wouldn't run anything wider than a 10" wide rim. I am running 35x12.50's on 8" rims
the 14.50's are gonna rub either way IMO