On-road -
I had 31's on my rig when I bought it (aluminum rims). They did fine everywhere. On the freeway the jeep would downshift to get up hills but drove fine otherwise. Really not much noticeable power loss. I was getting 15-17 mpg.

I have 33's on my jeep now (bfg at's on steel rims). Freeway is really not feasible. I have to run at about 3,500 rpms to maintain speed (65-70 mph). Country roads running 55-60 mph is very manageable. I can feel some power loss now; nothing very big, but its noticeable. I'm getting about 11ish mpg and can squeeze about 14 out at 60mph.

Off-road-
The type of tread is a bigger determination of how your rig will perform off-road than is tire size; especially, between such close sizes. You gain a inch or two of axle clearance. Your tire width may or may not change with a bigger tire which also may or may not help your jeep's off-road performance.

Overall-
If you drive freeway, un-geared 33's is going to kill you. It definitely gives your rig an aggressive look and stance (especially at about 3 inches of lift). You'll most likely have to trim which some people like to avoid. If by M/T your referring to tires like Duratracs, MT/Rs, or STTs, you really won't notice a mpg difference between them and an all-terrain. The only difference would be tire wear.

 

Is there a very big strength difference between aluminum alloy and steel as far as rims are concerned? I ask because I dont tow trailers or anything but when mudding I don't want excesses strain on something aluminum.

 

And I assume your gearing was stock with the 31's?

 

No difference in your application. Steel are preferable when rock crawling as if you come down on them hard they just bend instead of shattering, and you can bend them back. Outside of this it doesn't really matter.

 

I'm at three and a half inches with a little sag and I'm running 31 sometimes it's not about how big your tires are it's about how your suspension setup works I can go a lot of places that people with 33's are going simply because I have JKS discos x flex uper n lower ca and the right offset on my rims I can stuff the tire in that fender well with no trimming just as well as anyone who has 33's and trimmed the crap out of thier rig I Drive it to work every day of the week and it doesn't really ride all that bad I will admit the 33 will look a lot coolerbut if you're looking for off-road capability you can do a lot with 31

 

I originally went with 31's on 3.5 of lift and instantly wanted to go up to 33's for an even more aggressive stance. After a month, I purchased the 33's and did some minor trimming to keep from rubbing. It looked great, but my mileage fell to 10mpg with stock gears and a daily interstate commute. I removed the 33's after only a few weeks sold them and used the money to pick up Aussie lockers front and rear. After trailing and daily driving both setups, I definitely like the 31's more.

 

I know most people will think I am crazy, but my last XJ I installed a 6" RC long arm kit, SYE, Tom Woods Drivelines, and ran 35's still needing to trim the fenders a couple inches.. I had 3.73's with the 35's and it ran like a champion. It is a Jeep bro, not a Lexus... I could still easily do 85 on the highway and even pass people when I felt the urge.. Of course that was a daily driver as well.. I think it is all OP. If your going to be running it hard then YES regear with anything around 33 and up.

I also had a 2004 Jeep TJ that I ran 33's on with stock gears for 8 years. Ran great and never a problem.