Its roughly 15% so on a tank where you get 500km you can add around 75km to the distance travelled.
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I ran a 3inch lift on my GQ wagon. King springs, matching shocks, extended brake lines, castor correction bushes, all included in the kit for around $800 from eBay. Disconnected the sway bars and adjusted the brake proportioning valve and fitted adjustable pan hard rod. Ran 35's without dramas. You might notice an increase in fuel consumption, for 2 reasons. 1, the tread design of your choice of tyre, in guessing somewhat aggressive and 2, the final gearing of you truck with the bigger tyres is altered. Even though each revolution of the drive shaft will put you further down the road, more power is needed to move you due to the higher gearing. Not unlike trying to ride a mountain bike in low gear compared to high gear.
The other thing to consider is the offset of the wheels you're choosing, as this will determine whether or not you'll experience tyre rub, especially when turning.
Hope this info is of use to you.
2" lift and 2" body save you a lot of money
I have a 4 inch suspension lift and have trimmed the back of the front and rear guards. with 35x12.5 r 15s they still scrub a little when flexed.