76 Chevy with speedometer reading 10 mph low

[Kimper]

Heres my issue fellas, my speedometer reads 10mph low no matter what speed I am at but my odometer reads right on when I check it doing mile test with my GPS. Does this mean my speedo gearing is off at the tranny or my speedo is junk?

[1967KaiserM715]

Because your odometer is right, and it gets its reading from the cable your Speedo gauge might be the culprit

I believe Speedos needles need to be indexed to read right meaning there is a short section, say from 0mph to 7mph where the needle only moves a little bit-this bit takes up any "slack" from the cable. 

At least that is my understanding, which is why it is not recommended to remove the needle unless necessary

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[Kimper]

So it sounds like I need to pull the needle off and re index back on the speedometer and then check it, repeat if necessary..

[1967KaiserM715]

Different car, but on the early 80's Mercedes, there was a tick mark on the other side of the rest pin, placing the needle to point to that then moving it back to the other side of the pin would index it correctly

Maybe there is a similar way on these speedos, haven't worked on these trucks long enough to know all the ins and outs


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[philo_beddoe]

speedometer calibration.
My speedometer reads exactly 10 mph under, why?  Pretty sure its factory.
http://forum.73-87chevytrucks.com/smforum/index.php?topic=31810.msg267649#msg267649

[Usareke]

Mine reads 10 mph higher than I'm actually going.... _-_ from what I've heard the plastic gear can get worn out and cause this or the gear reduction box on the tranny tail shaft could be out a wack

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[Kimper]

Thanks for the info guys, I guess I will start with the gear and see what that looks like. I know it has a rebuilt Th350 in it but I am not sure if it's the one from the factory..