Speedometer issues

When you get into it you'll see a magnetic gyroscope like assembly. One part spins inside another. Look for shinny rub marks on the outside of the drum shaped part, evidence that one part is contacting the other. It might be that cleaning the hardened contaminated grease crud from the square input port and routing the cable into it as straight as possible will help but that's only if all the wear in in that shaft/bushing fitment. If the wear is in the two small bushings that the drum/needle post assembly runs in, send it out for restore or gamble on a used one.