Hi All:
I've got a '68 with a speedometer issue. When I drive the speedometer wavers between 10 or so mph more than I am going to 10 or so less than I am going..back and forth..back and forth...I have the gauges out now and the cable is loaded with graphite so I don't think it's a lubrication issue...could it be loose at the tranny?? I have a small tranny leak and maybe the cable is not in all the way?? I checked the cable and it does not look like it is kinked at all....
Thanks!
D. Schribert
'68 convertible

Usually this is due to a kink or dry spot in the speedo cable. I would remove the Cable Assembly, take out the internal cable and inspect it carefully for worn spots that show it's dragging. Then run cleaner/solvent through the cable jacket and blow dry with air. Lube the cable fully with GM lube and reassemble and road test (or run with a drill if car is not drivable).
If this doesn't fix it it could be the cup dragging on the magnet in the speedo head, time for a trip to a good speedo shop.
Hope this helps;

To take Vintage's suggestion one step further, test the speedo head with a short piece of cable and an drill motor to look for smoothness.

But, I have found boucy needle due to a bent or kinked cable. Replacement is needed.