My 1942 speedometer occasionally makes a light grinding noise and the needle jumps around for a few seconds, usually at lower speeds (25 MPH). It reads accurate according to my GPS and I have definitively ruled out a cable problem. Is the speedometer itself serviceable? It acts like it may just need lubricated but I'm not sure how to go about that. Any ideas?
I don't know if this will work on your speedometer, but it might be worth a try. Some years ago when my '49's speedometer was acting in a similar manner as yours, I unscrewed the knurled nut that fastens the cable assembly to the speedometer head and squirted a small amount of oil into the head. It cured the problem.
Had the same problem with my 55. I had to remove the speedometer all the grease was hard as a rock removed all the old grease re lubed and has been working great ever since.
Keep Cruzin,
Bob Kielar