You are correct.  The cable from the transmission that screws into the back of the speedometer drives both the speedometer and the odometer.   If you remove the speedometer from the dash you will find the problem.  In a '56 Ford, the mechanism is entirely mechanical and fairly simple.  You should easily see the problem once you have it out.  Some of the parts are plastic.  You either have a stripped gear or something is disconnected.

On my '56 Fairlane I just did my oil change and noticed after a decent 2 mile drive couple weeks ago where I found a horrible spark knock and transmission falling out of gear, I realized my odometer is still sitting at 55,000.5 but I have photos from before the car was driven that is reading 55,000.6

Im curious what could cause this?  I don't know about the '56 gauge but I know newer speedometer gauges uses a worm gear to spin the odometer while the needle moves through magnetic interference as the cable spins the assembly which spins the odometer worm gear.

Im wondering if this is something that could be simple to fix or if it would require sending the whole unit out.  if I send it out then I would probably pay for a full restoration to make it like new.


1956 Ford Fairlane Town Sedan - 292 Y8 - Ford-O-Matic - 155,000 mi