I had received some info on this several months back but can't seem to locate it.
My car was a factory 12-bolt, 3:31 gears and I just pulled out the speedometer gear, red w/21 teeth, original Powerglide.
I found elsewhere that a 4:10 had 25 teeth.
I have changed out the gears to 3:73. Can I deduce then that the new speedometer gear would be 23 teeth?