Gas mileage goes down, I have run 4:11/4:56 gears several times in the past. It is simple the engine is turning faster at all speeds, faster means more fuel to feed it. 4:11's will give you a much better low end great in the woods, but also consider gear contact surface. With the smaller pinion gear you have a weaker link in the drive train, I blew more 4:11 and 4:56 gears then anything else. Take my word I was not easy on any size they all got hammered. With gas being what it is you want to consider transmission gearing if you have stick, that way you won't jack up your gas bill on the road.