Is the hex in the speedometer driven gear the same size as the hex on your cable?

With the cable removed from the transmission, if you turn the cable does the speedometer move?

Compare the AOD gear and the gear that was on the Fairlane originally. Is the hex socket the same style & size on both gears?

Put the original gear back on the speedometer and try that.
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Scott
1963 Fairlane Mini tub 10pt cage New 408" C4 Canfield 195CC heads Comp solid roller Victor Jr. 9" w/4.11? gears Moser spool & 35 spline axles. www.marksullense85carburetors.com

Everything matched up. Just for kicks I tried to rotate the old speedo by hand before I took the electronic one out. It did nothing. I did hear a noise up by the cluster though, like a light scraping. I just assumed that I wasn't turning it fast enough by hand to make it register on the gauge..
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'66 Fairlane 500 2dr "Valerie"

Take an old gear and cut the teeth off and chuck the hexed part up in a reversible drill and try it that way (slowly).

It is possible that the cable is not all the way in the speedometer, or it may be a bad speedometer head or even a bad cable.
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Scott
1963 Fairlane Mini tub 10pt cage New 408" C4 Canfield 195CC heads Comp solid roller Victor Jr. 9" w/4.11? gears Moser spool & 35 spline axles. www.marksullense85carburetors.com

Some folks will remove the speedo cable from the speedometer when attaching the cable up to the tranny. Most cables have a stop ring on this end so the cable can only go back so far. Make sure the cable isn't disconected from the speedometer or loose. If it is turn it by hand and ensure it's seated in the gear at the tranny then reattach at the speedometer.