Speedometer Wobble - Diagnosis & Repair?

[PurpleBeeper]

70 Road Runner 4spd rally dash - Speedometer had been operating smoothly, but then I drove 100-110 mph (allegedly ). Now, if I get over 70 mph the speedometer needle moves wildly (80mph-100mph even if I’m going 70 mph). I “thought” it was my speedometer cable. I installed a brand new OEM (circa 1985 new in box) cruise control unit and two brand new cruise speedometer cable. I saw a speedometer wobble again right away about 20mph, so I immediately unhooked the top can’t Le from the speedometer and ordered a used speedometer onlne.
1. How can I diagnose my problem for sure?
2. What’s the deal with “clocking” the speedometer gear in the transmission? I simply bolted it in several years ago and ran it.
3. How can I ensure my new-to-me (used) speedometer won’t wobble too?
4. Anything else I should know?

 

[kiwigtx]

What’s the deal with “clocking” the speedometer gear in the transmission?

You need to know the number of teeth on the pinion drive.

Then rotate the housing to suit that number - choose the position that covers the number of teeth you have on the pinion in your car.

 

[kiwigtx]

Failure to 'clock' the position correctly will lead to wear on the teeth ....and plastic fragments traveling inside the transmission.....along with incorrect operation of odometer.

 

[Bird 426]

It's possible that the speedo head may need to be serviced. The speedometer in my '72 Dart jumped all over the place. Sent it out to Instrument Specialties and it works just fine now.

 

[PurpleBeeper]

It's possible that the speedo head may need to be serviced. The speedometer in my '72 Dart jumped all over the place. Sent it out to Instrument Specialties and it works just fine now.

Any idea what they did to it?

 

[Bird 426]

No, I do not. Winter is coming, maybe send it out. If the speedo cable and gear are good, it's the only thing left.

 

[Sixpactogo]

The issue is more than likely the speedo head causing your issue. They get dried out from lack of lubrication. My 65 Sport Fury developed the same issue. I pulled out the speedo cluster and put a drop or two of clock oil in it. It has been working fine since then. This is what I used.
https://www.ebay.com/itm/230561333986?