I looked into this thoroughly when I switched to 33" tires and 4.56 gears. The dealer wanted too much money to calibrate the speedometer and was a one shot deal. My programmer would not recalibrate the speedometer and no one could reprogram it. I was not willing to buy a new one.
I found the least expensive way to recalibrate the speedometer on a 2002 is with a Recalibrator from Dakota Digital. I searched hard and found one for $75. They are normally more than that but you can usually find a deal on them. The recalibration process seems complicated but once you understand, it not that bad. The nice thing is you can change the calibration over and over, if needed. I switch back and forth between 33" and 31" tires so it is ideal for me.
It is not programmed by tires size and gear ratio. You will need a GPS or a known distance to get it set right. With a GPS you can you can preprogram it if you know how much the speedometer is off or you can do it while you are driving.
It does require hard wiring into the wire that goes to the speedometer as well as for a source to power to it. The faint at heart when it comes to wiring should not consider this route unless you have someone that can do it.
Welcome Visitor! Please take a few seconds and 
