Marcus,
If you are in Austria, then you must really have a rig that stands out!
As far as the mileage: I average almost 18 mpg with a 3" lift, 36" tires, 4. 10 gears, and a leadfoot. Of course, a taller truck is going to be slightly worse than a stock truck, but I think you would be hard pressed to measure the change. My guess is on the order of 0. 5 mpg.