I think it's about 1.5" difference in diameter. There are calculators online that'll show you what it'll look like, old to new.
I liked the look of the 31s better and I think they rode a bit smoother, especially on rough forestry roads. Performance wise, I think the losses were negligible. Though your odometer will read about 10% slower than it's actually going and about the same fewer miles shown on the clock. There's definitely more rolling mass, which does affect acceleration and braking, but not much. I think the style (mt vs at), quality, and drive style have more to do with it than a 1.5" difference. Off-road, it's a billy goat either way but even an extra inch of ground clearance can go a long way.
I got the truck in 2007 with bald 235 street tires. Went to 31x10.50r15 BFG TA KOs and drove them all over the place while I was stationed in NC. Mud, dirt, snow and ice, Sandy beaches, etc. They were great. I moved back to AR and decided to get MTs. Went with Firestone Destination MTs in 31". They were pretty good, but a rough ride especially on the road. Handled snow, ice, and mud fine but gravel and rocks were tough and chipped the treads a bit. In Nov '16 I rebuilt the front end and replaced the MTs with 235/75-15 Destination ATs and have about 3k miles on them, mostly highway and dirt roads. I'm happy with their performance so far and I gained about 2 mpg back. But I didn't get back enough truck performance to write home about.
All of this being said, when this set wears out, I'm going back to 31s. Probably BFG ATs, though I've looked at a few other brands. I think that's for a different thread though.
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Great stuff. Thanks! Mine's a 1990 4 cyl single cab with the 235s on stock rims. Tires it came with seem fine but are old and the fronts are different than the rears--some BigO ATs. The KO2s seem to come in either size and will likely be my choice. I'm no mudder or extreme off roader so will probably just stick with the 235s.