Correct Speedometer Gears for Muncie in a COPO

[Jeff Murphy]

My speedo is way off fast vs. actual (80mph indicated is more like 62mph) with only a modest 0.7" difference in tire diamater from stock (F70x15 vs. F70x14 original). The car was in storage for many years so the speedo didn't matter, but now that I'm driving it a bit, I'd like to have accurate readings. I'm guessing that it's either the speedo cable or that I've got the wrong speedo gearing in the tranny, or both.

Does anyone have experience with selecting the correct circle (drive) and pin (driven) speedometer gears for a Muncie 4 speed in a COPO Camaro with a 4.10 BE rear end or know what came factory? It seems like most cars should have an 8 tooth circle gear, but they come in two different diameters, 1.76" and 1.84", steel and nylon. There is also supposed to have been a 7 tooth 1.76", but some Muncie experts I've talked to have never seen one. All the repro catalogs list a 22t "green" gear as appropriate for 4.10s (which would indicate a 1.76" circle gear) but don't say how many teeth on it or specifically which circle gear it mates to.

I'm hoping to get a definitive or consensus answer on what the correct factory matchup was to allow reasonably correct readings with a 4.10 gear and stock 26.2" diameter tires. I am also hoping that I can get to the right answer without pulling the tail housing off the tranny to look at the circle gear and count teeth.

Do you think it might just be a bum speedo cable?