mikeys68

Can anyone tell me the first and/or second things to check into to fix a speedometer problem in a 1968 fastback? The car was changed to a Shelby gt350 clone and has Shelby speedo and tach now (which is probably when the problem was created...may be an install issue?). The needle moves, but is very "bouncy" and incorrect. It sometimes doesn't move at all. Is this usually an issue at the speedometer end or the other? Any recommendations how to fix it?

Halz5

I would take the speedo gear out of the transmission and spin the gear, have someone looking at the speedo meter and see if it moves... you probably just have a bad speedometer cable.

gothand

... or a chewed up gear, or even worse, chewed up gear on the tranny shaft. Hope it is the speedo gear. BTDT.

DGA

Step one would be to pull the cable (from the top) and lube it .

Meanwhile make sure that the cable housing is routed in smooth curves.

Possible other problems

1. Cable rounded at the top

2 Damaged driven gear. If you need to replace be careful to get a gear with the same number of teeth.

3. Damaged drive gear in trans. Rare. Again get correct tooth count.

LCC

In automatics if you have a old style speedo gear (short nose gear) in a newer trans (long nose gear) this is known to happen.

restomod66

Doesn't the color of the gearin the tranny mean something? I notices different color in parts magazines.

Soaring

We had this conversation some time ago, and Twister came up with the most comprehensive explanation I have ever heard.
https://mustangforums.com/m_1694016/...tm.htm#1694243

Aussie66Fastback

good answer DGA!

You may also need to lube the back of the speedo itself as that can be where the sticky problems are as well...a little bit of RP7, CRC or equivalent.

I had to do this a few times on my Fairlane to get rid of 36 years of dust, grit, grime, rust, dead bugs and whatever else, but in the end it worked. Now I just have to get the right number of teeth to make it accurate again as I had changed diff ratios.

good luck with it