General full is 3/4 of tread width is acceptable rim width.
Ie 75% of 12.5 tread gives you 8" for the rim width
I never had problems running the 13.5 iroks on a 7.5" rim; just adjusted pressure accordingly to get even wear.
Or you could look at the tire manufactures web site for recommended tire width. Different tires have different requirements. My 37x12.50 were on 8.25 x 16.5 rims, and probably would have been better off on a 6.75 because the width at the bead was really narrow to the point of being hard to mount. Where as most tires that size on a 16" rim would recommend a 10" rim. 9.75 is the next step up on 16.5" rim, and I think getting them mounted would be near impossible. Not all tires are created equally.