Changing the speedometer driven gear to fix speedometer speed-

Inside your transfer case or tail shaft of transmission you have a speedometer gear that provides output to a cable or electronic speedo to tell your speedometer how fast you are going. From the factory this is set up correctly, but when you change your tire size this calculation becomes off and the speedometer becomes inaccurate. 

If you remove your speedometer driven gear from your transmission/t-case, you will find there is a plastic/nylon gear on the end of a shaft with numbers at the end of it. There should be 2 numbers. One is the number of teeth on the gear. This can range from 16-33. The other number is the drive gear inside the transmission/T-case that drives this gear. This is usually 6, 11 or sometimes 10.  By using a speedo calculator, you can figure out what gear you need to fix your speedometer issue. There are many available online. Here is a link to one offered by JEGS.  What you need to make this work is you need to know your rear end gear ratio, your tire size and the number of teeth on the drive gear inside the transmission/T-case (hint: it is stamped on the gear). 

In the example below the rear end ratio is 5.29 with a 36" tire. The Tcase has an 11 tooth inside it.  This is calculated out to a 33-tooth needed. Mine had an electronic speedo with a short shaft and the 33 tooth only came on a long shaft.

The plastic/Nylon gear is held on the shaft by a knurled section on the end of the shaft. By supporting the bottom side of the gear, you can tap the shaft out of the plastic gear. I put a large washer under the head of the gear for support and used a 1/4" punch and tapped the center if the gear to knock the shaft out. It took about 6 or 7 taps with a hammer, and the steel shaft was free of the gear. I did this to the new gear and the old gear. Re-using the original shaft with the new larger 33 tooth gear. The shaft can be tapped into the new gear until it seats all the way down. The knurled section of the shaft holds the new gear tight without any play. 

This fixed the speedometer issue.  

Changing the gear inside the transmission/T-case is an option but that is considerably much more work and there is not a variety of gear options. Changing the plastic gear can be done start to finish installed in 30 minutes.

https://www.jegs.com/tech-articles/how-to-choose-the-correct-transmission-speedometer-gears/