it's defiantly a good ideal to re gear but not necessary for 35s. My gas mileage is about the same as stock, if not slightly better after I used the Superchips tuner. I am running 35s with stock gears. However,, I entertained the ideal of re gearing but it's not a high priority for me. My jeep drives better than I thought it would.I would venture to say that anything at 33" or bigger tires you need to re-gear.