Speedometer cable

I wish I still had pictures of worn out speedometer parts to share. There is a magnetic propeller turning inside a drum or bell. When the tiny bronze bushing wear enough the magnet part starts touching/kissing the edge of the bell. That scaring and the egg shaped bushings is what I had pics of. Anyway... the slightest touch/kiss at very very low speed will toss the needle from 0 to 10 or 15 mph or more.
Its theoretically possible that at a high enough speed the needle reaction disappears. Not that it isn't still happening, only that the effect isn't visible. Needle return spring tension over rides the effect. Some theory and speculation on my part. I have seen examples of the kiss became a grab and hold. The needle ends up laying at the bottom of the screen.
If you can't cure it at the cable, take the instrument to the workbench and look for its fault. Then replace it, or have it rebuilt.