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I have the 8 speed I figured around that my 17 Silverado gets 16 on 35s with a 6 inch liftbeing a 14, you have 6 or 8 speed? makes a difference. im lifted 4" with 35x12.5 8hp70. no highway and i can get 15 to 15.5, sometimes close to 16 hand calc. got a full tank averaging 17.1 on evic once. i get better gas mileage on shell 89 for some reason. i have tested this on several tanks. consistently close to 1 mpg better with shell for some reason. highway should be even better.