Speedometer - any difference in gearing within the gauge?

[Nevada51]

As the title says - is there any differences within the gauge gearing, or does it just come down to the gearbox cable gear? Background for context:

• My 1964 c10 speedometer (ink stamp dated April 1964 on the back) was having an issue where the needle was 'bouncing' and making a 'ticking' sound when I was driving, and as such my speed measured was +/-15 mph when doing 55 mph. Turns out the vertical odometer gear in the unit was binding and releasing which was causing the bouncing/ticking.
  
• Rather than just replace the gear in question, I bought another new gauge unit (ink stamped May 1965; not sure what model truck it came from) and installed it. It fixed the bouncing needle problem, but the speed it measures is now around 25-30% too low (eg when I'm traveling at 35 mph, it measures 25 mph).

So to my original question: could there be any gearing differences in the speedometer unit itself? I wouldn't have thought so, but despite the needle bouncing with the original speedo, I thought the speed reading was about right (the bouncing was far less at lower speeds)....but maybe I was imagining that!

For the record, it is an original 3-speed manual gearbox and the tire size is 6.70x15, so everything is factory stock. I didn't touch the speedo cable gear in the gearbox.

I could always pull the cluster AGAIN, replace the specific odometer gear in the original gauge and see if that answers my own question...but that will be a PITA (the speedo cable nut is an exercise in hand contortion I'd rather not go through again!), so I'd rather have a definitive answer before doing so.

Any thoughts appreciated!