With the bigger flares definitely go with the lower backspacing IMO. I agree that 12.5 wide or less tires look worse on 15x10 than 15x8 but its another option and maybe he likes that look better.
15x10 people say 12.5 tires look to stretched on 15x8s so its all personal preference.
Are your options still either a 3.75 or 3.25 BS? Or do you have other options your choosing between?
Me personally I would choose the 3.25 out of those 2 since you have the bigger flares but again everyone is going to think differently. 06Ford gives you a very good idea of what the 3.75 backspacing looks like with the bigger flares and almost even with outside of the fender. Remember that we were running 33x12.5 tires so your tire will only be 11.5 inches wide if not actually a little less because sometimes the measurements aren't exact to the real life measurements.
So if you want your tire to be about even with the outside of the fender flare then go 3.75. If you want it to stick out a small amount more, go with 3.25. It's up to you.
2006 Ford Ranger Supercab 4x4 XL. 4.0 5spd. To much to list-SOLD!
Ill be back