More back space will bring your tires in to the
fender, less will push them out of the fender
I would say get some 7" wide 3-1/2" backspace
for a set of 31" tires, some will say get 8" or even
10" wide wheels, but IMO those are too wide.
I run a 33 13.5 Swamper on a 8" wide 2 or 2.5"
backspace wheel, I like the wheel more narrow
to help keep the tire on the bead at low pressures
Wild