Larger tires decrease gas mileage?

[Bruce Willys]

Yes, they do. Recalibration will make the speedo and odometer more accurate, but you're always going to lose some gas mileage going to a heavier tire with a higher rolling resistance. Look up unsprung mass in relation to wheels/tires, there are a lot of good articles out there. Adding weight to your tires and wheels makes a significant difference in performance, more than most people realize. In my experience, I've always lost at least a couple miles per gallon going bigger/heavier especially with the lack of flat terrain in my region.

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[olmectech]

Do a spare tire delete and gain 1 -2 mpg back. That spare hanging off your tailgate also impacts how your Jeep handles.