Ok, on my 77 F-150 4x4, 4sp we went out cutting wood, had it in 4 low the majority of the time, backing up pulling logs up the side, on the way home the speedo in erratically bouncing ALL over,,, 0 to past 100 to 20 to 80 etc.
Odometer works fine, once I am up to about 40 or 50 it kinda settles down. Do I need to find a new speedo?

 

No. I would venture a guess that your speedo cable was damaged, or your gear that goes onto the end of the cable to turn it was damaged. take it off at the t-case and turn it by hand, If it binds up, you need to replace your cable. If the gear has wear, or missing teeth, replace it too.

 

Guess I will be climbing underneath it after the game and check it out.

 

Mine has been doing this same thing for about 5yrs, only does it about 2-3 times a yr, usually when its real cold. Sometimes it does it for a few miles, sometimes just once or twice. Lubing the cable did not help mine(was my 1st diagnosis). I pulled the cluster out a coupla months back & replaced the crusty plastic(light bulbs were falling out), & I put one drop of sewing machine oil on the speedo after cleaning it. It hasn't done it since but it hasn't gotten cold yet either.