[Accelo]

There are two different worm gears possible on the tailshaft. The 1.84" OD gear uses 17 to 22 tooth pencil gears that are .810" OD. The1.76" OD gear uses 22 to 24 tooth pencil gears that are .870" OD. The 20 tooth blue, or 21 tooth red are both used with the 1.84 OD worm gear. The smallest that will work with the 1.76" gear is the 22 tooth green. If you have a 1.76 OD worm gear on your trans tailshaft and the 22 tooth green gear reads too slow then you have the wrong worm gear on the tailshaft. Only way to fix that is remove the trans, pull the tailshaft housing, the gear, and press on another gear. Or, i believe there are aftermarket gear conversion devices that allow you dial in a change although I've never used one of those.

This is the information for the typical 60 era speedometer gears.
If you send me your tire diameter, or better yet the revolutions per mile from the manufacture, and the differential gear ratio I will do the gear math for you.
Remember the GM calculations were typically off some. The error was typically on the fast side for liability reasons.