you can run 30's with no real problem. i've also heard 31's will work, but there will be rubbing when off roading and rubbing at full turn. if your springs are old and sagging i would stick with the 30's
I had a 90' YJ Islander in college. I also sold tires at a tire dealer at the time. I put 31's on and experienced significant rubbing. The Islander has a little more of that rubber molding around the fenders, I think. I took the 31's off before I left the shop and put 30's on. I used MT's and ran all over Colorado mountains in the summer and North Dakota fields in the spring / winter. No problems. I would go with 30's again for that year.
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