[Hersbird]

I think the biggest hit of big tires is the height. Know the manufacturer doesn't have to publish differnt MPG ratings for off road packages with bigger tires or other add ons that obviously will effect real world MPG. So a Rubicon Gladiator supposedly gets the same MPG as a basic sport, or a Sasquatch Bronco is the same as a base as long as the engine and transmission are the same. Of course that's not the case in the real world.

Anyway big tires and the also added lit kit needed for big tires is like adding a big cargo box on the roof. Don't think more air will just pass undisturbed below the truck because the underside of these things is just an aero nightmare. So with a lift kit and big tires you hurt any factory tricks to improve the coefficient of drag like say a front air dams proximity to the ground, all while adding overall frontal aera for significantly more overall drag.

PS I'm in western Montana and used to have a 2004 2500 5.9 6 speed Cummins 4wd and it was plain terrible in the mud sand rocks and snow compared to my stock 2009 Jeep Commander with the Quadradrive 2 and e lockers front and rear. Now I've added a 3" lift and 32" tires and it's crazy better than the Cummins off road. The Stock Rubicon Gladiator already has 33" tires and lockers front and rear and will be much better than your Cummins. So get and Ecodiesel and with some basic ecomodder tricks you will beat whatever EPA rating they have which I think is high 20s MPG.