Speedometer issues

Hey electronics gurus, I have a 92 iroc rt that the speedometer won't go over 20 mph, the odometer works perfectly and the door locks automatically lock when the car reaches 20 by Gps, the speedometer is barely moving when they do, so I know the speed sensor is good. Can the actual motor that drives the speedometer be bad? I have used the cluster out of my 90 vnt, and the speedometer works but the speed is a little off, but I think it's because it's a 140 speedometer instead of the 160 iroc speedo. What are some thoughts? Thanks in advance.