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!

[Short 3/4]

They should all be geared the same. All changes happen at the transmission. Could it be possible that your “new” speedometer is dragging (not sticking) a little? I do know that a speedometer shop can fine tune it by manipulating the magnets in the unit (not sure what the exact science is, but I know they don’t change gears). In the old days there was a speedometer shop in every town, but now there are just a few mail order places. I’ve sent units away to be calibrated and it’s not a big deal. They also clean and lube them.

I agree getting to the cable nut is no fun, done it too many time and it never gets any easier! Good luck.

[bhap]

If the speedometer needle is smooth and steady and consistently wrong, I’d take the win and adjust at the transmission. Depending on your trans, there are different tooth counts available or you could do the math and order a ratio conversion box based on your tires and differential.

[theastronaut]

Check that the odometer is accurate since its mechanically directly connected to the trans. If its correct then you can tell if your speedometer is calibrated correctly or not. If its not correct then adjust the speedometer drive gear in the trans to correct the odometer reading first, then have the speedometer calibrated.

[Nevada51]

Thanks everyone, and a good point you raise Short 3/4, perhaps it is dragging. I think the 'easiest' solution for me might be to pull the gauge again, replace the odometer gear from my 'new' unit into the original gauge...and hope that it isn't the source of the dragging! I'll do it eventually, once my cable nut PTSD has dissipated! If it wasn't for that, the job of pulling the gauge cluster and reinstalling would be pretty easy.

Theastronaut - that's a good idea re checking the odometer reading, so I'll give that a shot. As a complete aside, I see they've finally made a movie using your name..but I doubt it features any pick-ups in it