Not sure why you have to remove the speedo but the entire gauge cluster will need to be removed and disassembled to remove the temp gauge. If it is still functional you need to feed the probe and line back into the cab and remove it intact along with the gauge cluster. The trim ring on the cluster is crimped on the edge and that is what holds the cluster parts together. You have to un-crimp it around the cluster circumference and then the trim and glass can be removed. You have to open up the cluster this way because the temp gauge will have to come out thru the front. Everything goes back in the reverse order.

This is not necessarily easy but the only way I know of to replace the gauge.

Chuck