I remember when I first got my jeep with the Stock S**T tires, it got ok mileage but had no traction.
Then I get some heavy Kevlar tires, lift it, add a push bar, roof rack, additional skids, gear ++ oh oh, bye-bye fuel mileage.
Re-gearing will probably get you an additional 3-5 mpg depending on your tires, accessories, weight of your Jeep.
I put my jeep on the scale. The KJ curb weight is around
4033 lbs. KK is 4269 lbs.
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Add 2 extra persons, all the camp gear incl heavy coolers, extra fuel etc, add another 800lbs easily.
Keeping your Jeep on a diet helps regarding fuel mileage!
I did not buy the Jeep for fuel mileage though. I have a real 4x4 that gets me around in style.