31 inch tire 92 auto trans 2wd i want to get the best mileage im not doing any rock climbing but i do go in the mud every once in a while to get to a fishing hole. also how hard is it to change the gears on 8.25 limited slip

 

With 31s, an Auto tranny and 2WD, I'd leave the stock 3.55s in it. 4.10s will get you a little more umph but, I think they would actually hurt your mpg for your application.

 

I would go 3.73
my GF has an 05 TJ 4.0 6spd with 3.73's and 30x9.50's and I have to say it is absolutely the best factory gearing of anything I have ever driven. it will ***** and git around town and on the highway it still gets decent mpg. I have gotten 21+ mpg out of it a few occasions.
31's on the same gearing would only improve the scenario as I see it. the jeep has enough gearing with 30's that jumping to 31's wouldnt be noticeable at all but adding that 1 extra inch of diameter would help highway mpg a touch and might even get her rig into the 22-23 mpg range.

go for 3.73's it will help your around town driving and mpg by you not having to use as much skinny pedal to get moving and on the highway the 31's are tall enough to overcome the RPM jump the gearing will add