Usually wrong speedometer was right for a few minutes

[68 TT]

Originally Posted by EARNHARDT#3

Try lubing the cable first.

I second that. I have fixed several misbehaving speedometers over the years with just a good cleaning and grease job on the cable. Soak the sheath and cable in mineral spirits for a few hours then blow it dry to remove any debris or crud. After that use white lithium grease on the cable then put it back into the sheath.

You can also spray the speedo mechanism with electronics cleaner to remove any crud that may have accumulated on its moving parts.