I remember reading somewhere, most likely this site, that a good rule of thumb is that the widest tire you can fit on a wheel should be 50% wider than the wheel. That means that on an 8" rim you can fit a 12" tire. so 12.5 should be fine. also, youmight want to check out 285/75/16 for a tire size. these tires are about an inch taller than a 33" and about an inch narrower.