So I know this post is really old post, but I had the same situation and I am certain someone else will down the road so here is how I fixed it, the cable was known to be good and so was the cluster, and the speedometer driven gear was also known to be good, in my case I was swapping to a different transmission I had gotten from a guy that had it rebuilt, it was a km145 from a 4x4 mm, again I noticed that the speedometer gear was in perfect shape when I removed it, but whoever rebuilt the transmission installed it wrong, yes you heard that right, you can pop it into the transmission a few different wrong ways, but only 1 way is correct if you can spin the key inside the speedometer gear housing, where the cable is inserted on the transmission, then it is installed wrong you should not be able to spin the speedometer gear if it is installed correctly and the plastic gear teeth are properly meshed to the transmission shaft.