Speedometer gears in Transmission
 

I am working on a project and I have a problem I have a mech Speedometer drive installed on a 560 Tranny. Since the Transmission was installed my Speedometer has been tempermental. I removed tail of Transmission and found that both Speedometer gears are chewed up. They are both plastic and i can not figure how this could have happened. Is it possible that the smaller gear on the shaft that goes to the Speedometer cable could have been put on the wrong way. Has anyone with mech Speedometer knowlege ever seen this.