Cobber

BAH-HUMBUG!”
You used 2 “C” words…. One of which refers to wood butchery and the other to an obscene festival!

Back to the matter at hand…….

I was wondering how fast you could go in the lathe, obviously the faster you could get it to go the more accurate your fine adjustments will be. To optimal speed being a little above whatever the top legal speed is in your jurisdiction.
Maybe a simpler way might be to use an old speedo cable in a fast drill or a speed adjustable die grinder in conjunction with an optical tachometer to accurately calculate the rpm.
Another advantage of using the cable to drive the speedometer is that you are replicating the real world conditions.
The problem I can see by using the direct drive in the lathe method is that you are leaving out the torsional twist and frictional drag factors of the cable.
In regards to real world conditions, you would be better to rig it up in the car as that would take into account the frictional drag from all the various bends in the cable as installed.
The reason the speedometer needle jumps about at low speed is the cable and it’s various bends, at higher speeds it still suffers from fluctuations but the higher frequency sorta irons them out and the needle can’t react fast enough so as you don’t notice.
That then throws up the problem of getting to the speedometer in order to adjust it….. ah bugger it, I’ll just leave the bastard as it is!