I've run 31x10.5 tires on 6" rims for over 150,000 combined miles and find it a very agreeable combination... a 7" rim is a bit nicer and 8" is almost too wide, if you're going to air down.

However - running a combination specifically recommended against by the tire manufacturer will probably void your tire warranty and may leave you liable in certain legal situations.

I read on another forum of some guy who ran 31x11.5 tires on 6" rims and got very unfavorable wear patterns, but I had no such problem with 10.5" tires.


(edit - did the math on how much I've driven on 31's & 6" rims)