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I did a 2.5" Level kit (bilstein shock) and LT295/65-20's. (35 x 11.5) gas mileage suffered a little. Probably 2-3mph on the highway. it's hard to say for sure as the speedometer and odometer is off due to tire size by about 15%, and I never fixed it. More noticeable, I have the 5.0L with 3.55 gears and you can feel the difference with the bigger tire. I've considered a gear upgrade to make up the tire change.