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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.