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.