The drive gear works works with all 56-series and 60-series pinion gears. It has too -- it is built into the output shaft. It is not a separate drive gear like on the larger transmissions. So only a limited range of pinions gears are available, namely 16-20.

Test your drive cable. See if you can easily turn the cable with your fingers, while installed on the speedometers. If it's not easy to turn, that's what is chewing up the gear.

Tooth pitch is fine.

I figure there must be only a few different size pinions that could possibly mesh properly with the worm gear, as they obviously have different diameters but the centres are all in the same position. My box has a 16T - maybe the 18T was too large?

Thanks DD. The chewed gear I removed from a 56 and is running in a 60. Should be ok.

There are definitely some shenanigans going on though.

You see, I am also getting speedo readings that don't make sense:
Originally had a 16T with speedo running at around 115% of actual speed.
Swapped in 18T (the chewed out gear) which ran at 90% from memory.
Based on the 16T, the 18T should have been (16/18)*115=102%, right?
Swapped in a 17T, logically in between the two, but ran at about 87% !

I think I'm going to check the pitch and diameter of these gears.

No special installation procedure. It should push in very lightly. If you have to force it something is wrong.

Maybe the wrong gear was fitted? The 63-series and automatic pinion gears look similar but are a different size of pitch. They even come in the same colors.

56 and 60 series gears are the same to each other.

Are there any special procedures to observe when installing speedometer pinions?

I just pulled a pinion and noticed it was quite chewed out. Just wondering if I may have damaged it, as I just pushed it in when installing - do I need to pay attention to teeth position or something?