From his profile lools like he has an LJR, so he'll need an electronic speed corrector like the Speedo Healer or Yellow Box.
It's early and I missed that one! I went through the same thing and installed one myself. If it helps, here is a writeup I did when I installed mine:
After having this in the garage for a long time I finally got around to installing the HealTech speedometer calibrator from Blue Monkey Motor sports. The calibrator plugs into the factory harness for all 97 - 06 TJ and LJs, including either the 231 or 241OR transfer case.
http://www.bluemonkeymotorsports.com/products_ht.html
The installation was literally this simple:
- Take the SPEEDOHEALER out of the package
- Unplug the speedometer wire that goes into the transfer case
- Plug the SPEEDOHEALER into the transfer case
- Plug the factory wire into the SPEEDOHEALER pigtail
- Run the wire over the transfer case and up near the firewall (battery side)
- Follow the simple programming on the well written instructions
- Zip tie the SPEEDOHEALER wherever you want it to be
- Close hood and test drive
Note #1: Before doing the install make sure that you note, using a GPS, the speed that the speedometer is reading when your actual (i.e. real) speed is 60 MPH. Once you've done this you look up reading from your speedometer on the chart they provide. For example, my speedometer read 70 MPH, while my GPS said the actual speed was 60 MPH. I looked up 70 MPH on the chart, which gave me the "correction value" of 14.3%. I entered this number into the control unit and it worked perfect.
http://www.bluemonkeymotorsports.com/downloads/SHV4A_User_Guide.pdf
Note #2: The display on the SPEEDOHEALER control unit, pictured, only shows one number at a time, so you would then have to enter the correction factor in one number at a time. In my case, 1, then 4, then 3, giving me the needed 14.3% correction
Final Note: The entire install took me 30 minutes from start to finish. This included adding split loom tubing on the entire length of wire provided by HealTech. It also included more time than normal for running the wire from the transfer case to the firewall. It was more difficult in my case, because I have a 241OR case and ultra tummy tuck. This resulted in me not having much room to easily access running the wire. The total cost was $120 (I think it's more now!), plus a few feet of 3/8" wire loom tubing.