Speedo issue

[sellersrodshop]

Got the old truck out yesterday, first trip past the mailbox since last October. At even cruising speeds, the speedo needle wobbles over a 10 mi span.EG: crushing at 55 the speedo needle waves back & forth between 50-60. Sometimes it would level out for a few seconds, then start jumping again. Any ideas on where to start on diagnosing this?

[Accelo]

A common cause is cable wind up. Typically this only requires removing the cable and lubricating it. Another cause it when the speedometer gear in the transmission is worn. This can be the square hole in the plastic gear get worn and skips on the cable. Or it can be gear tooth wear. I have included a picture of a gear with worn gear teeth.

['68OrangeSunshine]

Lubing the cable is easy, and if it doesn't work, then you know you need to go deeper.
Disconnect the speedo cable from the back of the dash. You also disconnect the connector at the transmission.
[I had my dash off, so it was even easier].
Get a tube of graphite lube dust.
Pull the cable all the way out and work the dust into the cable by pulling it thru some dust in your palm, until you get back to the end.
Attach the tranny end. Twist the cab end a little to feel that it's seated, then plug it back into the Speedo. That took a lot of the swing out of my Speedo needle.