my mods are sorta lke yours, but I dont even calculate anymore, when i use to though i got around 12-14 around town and never do any real highway driving. my mileage when it was stock around town was about 19. with exhaust and sct beating on it went to like 17-18 no real drop there but when I did the 4" pro comp lift with 35s is when I really notice that it went down.
you will need at least a 4" lift to clear those.. but a 6" would be better so that they dont rub.... unless you get cut out fenders.... those are the exact same tires im running on my truck and i have a 8" lift.. here is a pic
I've got those exact tires in 35x12.50R20 on the OEM 20" wheels. 4" lift. I get 14city/17hwy. They are quiet, nice tire. However they are HEAVY. They have some very deep treads. I think the issue with running anything bigger than 35's is rubbing the Radius arm. You'll need aftermarket wheels to stick them out more, which causes you to need a lift. Just generally speaking, with OEM wheels 35x12.50 is as large as I would go. I have "slight" rubbing on my Left (not on right) radius arm with 35's FYI.
I Have a six inch lift with 37's and mine rub a little...... Mileage with 20's and 37's is terrible but the company buys the go juice. Deals on used tires aren't always available and my new ones set me back almost 2k. Lots of fun and lots of "attaboys" will cost you.
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