To take this one step further..I have 17" dodge wheels with 4 1/2" back spacing. If I put the 31 X 10.5 will this rub?
Probably not on the outer edge of the rear wheel opening. Not sure about the shocks, but I think there's enough clearance for the 4.5" without trouble. Someone will know for sure.
But it might rub that little lip of the inner front fender skirt. Certainly has a good chance of rubbing the radius arm too.
But the good news is that neither of those is a dire situation. The skirt can be trimmed without anyone ever noticing unless they're intimately familiar with EB's, and the radius arms don't hurt tires very much. Just rubs some paint off the arm and makes a gawdawful noise!
And it might not even rub the lip at all. It just "can" in some situations.
...If a 10.5 fits the taller wheel should reduce the sidewall bulge and get me more clearance I think.
Sidewall bulge is a hard thing to factor.
For one, the actual bulging of the tire with a taller sidewall is only on the ground, under the weight of the truck. Up at the top where the fenders might rub, there is no weight-bulge.
And the "bulge" that most talk about from a narrower wheel is not actually a bulge at all, but a visually narrower bead rolling the sidewall rounder a bit. But it's not in any way pushing the sidewall farther out.
The measurement of a tire's tread and sidewall on a 7" vs 8" wide wheel is absolutely minimally different. And it does not make any part of the tire wider by pulling the bead farther in. Just makes it rounder.
But given the same back-spacing, the narrower wheel does move the overall location of the sidewall inward. Away from the fender lip.
Hence narrower wheels giving you more leeway typically.
Again though, it all comes down to backspacing no matter what width the wheel is.
Paul