Ok, I have been down this road before on my mustang. Last time, a year ago, I fixed the problem by replacing the driven gear at the transmission. The gear had excessive wear. It started doing it again recently. It tends to hang up at 20mph but when I shift into the next gear it jumps up to where it should be but wobbles a little. Anytime I accelerate moderately the needle goes backwards and then jumps forward once the rpms drop.

I replaced the cable, speedometer, and the driven gear once again even though the one that i installed a year ago still looks fine. The only thing that is left is the drive gear inside the transmission. I can't tell much about it by looking at it through the hole. This is in a 4 speed top loader.

Any help with this would be greatly appreciated!

 

the drive gear is bullet prof.
i don't know how to verify number of drive gear teeth by looking through hole.
bet you can figure out a way though.
wish they all had same number of teeth.
i would decode top loader build code numbers.
if lucky it will have original shaft w/ original # of drive gear teeth.
correct driven gear will last for decades if cable is not routed in a tight bend.

does your speedo give correct indication vs calculatated teeth?

how tight is your cable bend?

what is your cable routing path?

driven gear teeth may provide clue.

any pictures of gears and cable routing?

 

My money is on the speedo cable itself.

Even if it looks "good" no kinks and run correctly, you need to lube it. There's speedo cable lube at your local auto parts store to keep it from binding internally.

 

my cable started complaining.
i inspected and found the headers melted coating.
i lubed it down w/ lucas few years back and it is doing pretty good.

i didn't know they made a special lube.
is viscosity similar to gear oil?

 

best thing top use is Balistol or any gun oil. anything that doesn't turn into glue a while later

 

.....wonder about dry black graphite powder lubricant...
it can't dry out...

 

mine was doing that a few years ago. it was the speedometer

 

Hart has replaced speedo; hopefully new one works right.

my speedo was doing that and i put couple drops wd40 on it and has been good ever since.

kinda fun to play w/ it on the bench and test w/ drill.
make sure you rotate in correct direction.

 

I replaced everything associated with the speedometer on mine including the gauge itself about 2 years ago. A short time ago it started acting up and it ended up being the cable needing lubrication.

 

The best lube for the speedo cable (for your guns too) is a 50/50 mix of ATF and kerosine (by volume). It is also a great penetrating oil, tapping lube and vacuum leak detector.
Dry graphite will turn to wax from any oil or water trying to mix with it.
Because the speedo shows the correct speed until you begin to accelerate and then it slows down you have a problem with something slipping under acceleration. The gear (if worn) may be slipping between the drive and driven gear, the cable may be slipping in the driven gear, the cable may be slipping in the speedo head drive, or the cable may be pulling out of one of the two ends under torque load because it is just a tad too short. (a "tad" is just enough to frustrate you)

I had a spider build a web in my speedo and it would just not advance at all for the first few minutes then it would kind of work for a while but when I parked it overnight it would not work again. I pulled it out and found the web - cleaned it out with WD40 and then sprayed it down with penetrating oil (listed above) and the spider never came back - just a different weird problem that is probably not yours.