I did not regear for 31's and mostly its fine, but I did overheat my poor jeep trying to come up vail pass going 70mph. After cooldown I eased it up st 50 with no prob. But without regearing you definitely don't get quite as much out of it.

 

I have ran 3.55 gears with 31 and 33" tires the 31's were bearable but still a little hard on the trans/engine. When I swapped to the 33's they were a really big deal and had to be changed to 4.56's to get me back to stock. The 3.55s' and 33" tires on highway got about 10=13 mpg, 4.56's jumper it up 5 mpg and really helped the engine/trans.


Now I run 33's and 35's with the 4.6's and its still ok not a issue.