How much you speedometer will be "off" after a lift depends on what size tires came stock on your vehicle, what size tires you added, and what speed you are going. For me, going 65 on the highway was actually more like 75 or 80mph! That's a recipie for tickets. There are various devices out there that will fix the problem by reprograming your computer. There are extensive debates on JKO and JKF about which tuner is better, but I decided to go with the Superchips since it seemed the most widely used. When you are messing with your jeep's computer system, you want to use a proven product.
I have yet to keep track of mileage with and without the tuner. I've read it can improve your mpg's quite a bit. The jeep drives better after the tune, with less delay when you hit the gas and smoother engine operation. It also has a towing tune, which is good for me with my offroad trailer. Other jeep specific options include higher idle for winching and locker operation options.
