Here is a site to put your error codes into and you find out what they mean, saves finding someone with a computer to hook up.
http://www.ravens-ridge.com/JeepCodes.php
Speedo Gear chart
Changing Your Jeep Wrangler
Speedometer Gear
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After installing larger tires on your Jeep, it is important to change your speedometer gear as well. If you don't, your speedometer and odometer will be slightly to dramatically off depending on how much bigger you went. Fortunately, speedometer gears are relatively cheap, readily available at your dealer and take all but 5 minutes to swap out.
What You Will Need
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• Appropriate Speedometer Gear (See Chart on Right)
• 1/2" Socket or Wrench
• Ratchet
• Rag
Installation
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1. Determine what speedometer gear you need by using the chart on the left. Select what gear ration you have and look across to the tire size you just installed and where the two meet, that's the tooth gear you will need. The part number you will need to ask for will be 520676 + the tooth number. For example, if you need a 34 tooth gear, the part number would be 52067634.
2. Climb under your Jeep and locate the speedometer gear housing. It will be attached to the rear of your transfer case on the driverside.
3. Using a 1/2" socket or wrench, remove the retaining bolt and clip.
4. Firmly grab the speedometer gear housing and pull the entire thing out of the transfer case. You may need to rock it back and forth a bit to do this. Be sure to have a rag handy as ATF will drip.
5. Inspect the rubber O-ring on the housing for cracks or damage.
6. Hold the speedometer gear housing in one hand and the gear itself in the other and then carefully pull them apart.
7. Take your new speedometer gear, lube the shaft just a bit with the ATF in your transfer case, and then plug it into the housing.
8. Make sure the rubber O-ring is seated properly and then re-insert the assembled speedometer gear and housing back into the transfer case.
9. You will notice that there are a series of number located along the side of the speedometer gear housing. Select the appropriate tooth gear range you just installed and rotate it until it is aligned with the raised dot on the transfer case as shown in the pic to the left.
10. Re-install the retaining clip and bolt.
That should be it. Nice, fast and easy mod. Please let me know if you have any questions.