What a great question. No one could tell me, so I had to ask them down at Currie. This was my experience: I wanted to go from 33x9.50's mounted on stock Wrangler wheels to 35x12.50's mounted on steel wheels. 4 wheel parts sell steel wheels with 3.75" backspacing, and were pushing that even though I had been advised by the guys down at Currie to go with 4.5" backspacing. I was told they don't have thm, you want the extra width for stability, etc. etc. I wanted the tires to stuff inside the stock flares, therefor I wanted to stay as narrow as possible with the new tires. I was told to go tell 4 wheel parts to orde the 4.5" backspacing and I did. 10 days later I had the wheels I wanted(at lest I hoped I wanted). I mounted them up and they work beautiful! I have the same backspace to the rubber with my 35X12.50's on steel 15x8 wheels as i had on 33X9.50's on stock wrangler wheels(which have a backspacing of about 5.75" or so. I do touch the spring in a hard turn a little, but doesn't bother me for the trde off. I love my tires and wheels! 4.5" backspace is the only way to go for me, and I'm glad I stuck to my guns!
93yj, 4.0,ARB f&r,atlas 4.3, 4.56's,SOA w/rncho 2.5", MORE strng conversion, Tele pn hrd br,2" bdy lft,frnt shkle reversal w/buggy, 35X12.50 Pro Comp MT