Joe,

Here is what I recently learned through trial and error (and getting schooled on here)
1. Odometer is run off the “tit”/ male portion of the cable.
2. There is a shoulder looking part above the tit that connects to the speedo part through a magnet. If the magnet is week, or not seated correctly, speedo will not read correctly due to slippage.
3. Guys here recommended that I tested mine by driving a known distance (Ie 10 miles on interstate) and observe how far odometer travels. Ie mine traveled 8.8 miles so it proved mine was off by certain percentage. (I am purchasing gear to compensate in the future)
4. I am also using cell app for speedo to check MPH so I can accurately guess actual speed.
5. I had issues with my cable not seating correctly. You can either try reconnecting it to ensure it is back correctly or leave one end hooked to your transmission (other end unhooked) and have a passenger observe to ensure it is spinning when driving. (I both)

I know that this doesn’t answer your questions, but might narrow down the issue.

Chris

1951 Chevy pickup 3100
In the Stovebolt Gallery
Discussion in the DITY