It will affect it, the 12.5" wide more so than the 33". No one can give an semi-accurate guess on MPG without knowing what part of the country, or what country you're in.. as well as your driving style, type of gas you use, etc. I can tell you that our '16 with a 315/70/17 tire got around 15.5 EVIC average in fairly flat Oklahoma.
If gas mileage is a concern for you then I would leave it alone. One might also suggest that if gas mileage is a concern you may have chosen the wrong vehicle If not a huge concern then add your tires and enjoy them!
Glad to have you here!
Welcome to Wrangler Forum :welcome: :wavey:
The Wrangler is so inefficient moving through the air, that any other changes will have very little effect, except for forced induction or lower gears.
I would say the effect is so small as to be inconsequential. At $3/gal, your probably talking about $0.75/100miles at most. That's way cheaper than a pack of cigarettes or a bottle of Jack Daniels and you will feel a lot better after.
A forum community dedicated to Jeep Wrangler owners and enthusiasts. Come join the discussion about reviews, performance, trail riding, gear, suspension, tires, classifieds, troubleshooting, maintenance, for all JL, JT, JK, TJ, YJ, and CJ models!