97-06 TJ (non Rubicon) have a mechanical pick up. The way to adjust the speedo is to change the gear. Rubicons have an electronic pick up and the only way to custom adjust is by an add on electronic box. There is nothing to do in the computer in either case. The difference going to 31's is minimal and not worth it.
2003 TJ, 33" BFG AT, 2" BL, 2" BDS spring lift, Truck Lite headlights, Homemade sunrider top.