I am new here and I am new to owning an HMMWV. That being said, old fashioned speedometers are pretty easy to understand.
There is a hose that travels up from the transmission to the speedometer in the dashboard. It is held in place to the back of the speedometer with a round screw on fastener that entirely encircles the tube and has ribs on the side to allow for better grip. One needs to turn the fastener counterclockwise several times in order to release the tube from the back of the speedometer. Inside of the tube you will notice a long-arm wire that is square in shape. In order to replace the speedometer, you need to make sure that the square wire fits back into the square hole before you tighten the fastener that holds the tube to the speedometer. If you see that the tip of the wire is rounded, you may need to replace your entire speedometer cable. It is the square shape that articulates with the square hole that allows the speedometer cable to spin the mechanism and register with the speedometer assembly.