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

 

I just came across that information and then back to the forum to check on info for tail shaft gear and I saw your reply, thank you sir. With the way the gear is chewed up it's going to be hard to believe the transmission gear also isn't a problem. This may be just reinstall the old part for now, until I find out more about "pulling the yoke and the extension housing". I much appreciate the information, it's better than spending the money on the cable and drive gear to be disappointed and I've got a Garmin that gives me speed.

 

It actually might be worth taking a chance on a new cable and gear, I put a finger in the transmission hole for the gear and the teeth felt sharp to me. I couldn't feel a concave surface but I can't swear how well my fingers are calibrated. I tired to get a visual but space is limited for line of sight. Having now spent some quality time under the truck its obvious it would benefit from a tail shaft seal replacement. there's a bit of seepage and I dried it off yesterday.

Just for information, the cable that came out measured at about 69 inches, Dennis Carpenter, has the cable the looks most correct at 64.8 inches. I checked the routing yesterday and the shorter cable should be fine.

Dennis's cable has the rubber seal at the tip where the driven gear slides on, though it is missing the O ring for the wide part of body (but the space is there for the O-ring), Some cables I found don't have the tip seal and it's not that they are missing the rubber part of the seal, there just isn't any tip seal., I don't know if the tip seal is really needed, just that my cable has it. I can reuse the existing O-ring.

 

Going the conservative route I ordered the only the gear off Amazon, (the only one I found that included a new O-Ring), it looked exactly like the original, put it on the old cable, it even slipped back into the transmission easy, drove it around the block and speedometer works again..and smoothly.

With that proved out and since I've never changed the cable I'll order a new one from Dennis C along with a new gear retainer clip. The cable Dennis caries is the only one I've found with the rubber seal at the tip where the gear slides on but, it doesn't include the O-ring. Dennis is cheaper on the gear but was out of stock for the 18 tooth and couldn't say if they would be getting it back in stock, it looks like they have. I don't know if the seal at the tip of the cable (transmission end) is really needed, but the original cable has it.