On my F250 390 with AT, quite a while back the speedometer was bouncing at low speed and then just quit, never to return. I've got a Garmin GPS that gives speed so no big deal but I've done some work on the truck lately and decided to try and get it working again, getting something working always feels good.

The gauge cluster comes loose with removal of a few sheet metal screws and then there was enough room to get a hand in behind and loosen the speedometer cable. The foam firewall seal around the cable was hardened up, It was pulled out piece by piece which then allowed the cable to pulled through to the engine side. I'll get another or make one out of foam pipe wrap .
Disconnecting from the automatic transmission was simple, one bolt for a clamp that holds the cable in place, though it took some pressure to get the speedometer gear end to back out. The cable it's self, in my case, had one tie wrap and one clip holding it the installation path. Done. One note, after the transmission end is taken loose I did block the open hole and it's probably worth wrapping the gear in plastic wrap or a baggie to keep the teeth out of the muck as the cable is dragged out (I didn't, I will when it goes back in).

I was expecting to find a broken cable, but no, twist the gear end and the center steel inner cable spins. I hooked the extracted cable back up to the speedometer and turned the gear by hand and the speedometer moves. But the gears look just a might wore. The cable it's self is quite stiff, probably the same for a new one since the inner cable turns freely. so It looks like it will be a new speedometer gear for the transmission end of the cable, I've found one source, will look some more.

For the record, The cable that came out measures right at 68 inches, metal end to visible metal end not including the transmission gear. The transmission gear on the cable is 18 teeth and is actually marked with and 18 just off to one side of the retainer clip. To test if if the cable drives the speedometer, rotate the gear CCW.
The inner cable is obviously replaceable as the cable can be pulled at the speedometer end and it will back out and spin independently of the gear.

 

You better look at the drive gear too
The one in the trans
Stick your finger in there and see if the gear feels like the one you took out (all chewed up)
They get chewed up when the cable is hard to turn
You have to pull the yoke and the extension housing then you can replace the 17285 drive gear
Those are also color coded and have then tooth count printed on them IIRR
You can look in that little hole and inspect the gear and count the teeth
Gear should be flat not concave