# Speedometer gears



## histoy (May 22, 2019)

My '66 auto speedometer reads 5 mph higher than the actual speed. Is there a general rule about changing the number of teeth on the gear? In other words, if I increase the gear by 1 tooth, how many mph will that change the speedometer reading? On other cars I've worked on it was around 3 mph.


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## gmjunkie (Jan 1, 2019)

It's really hard to go just by the number of teeth because there are so many factors that affect the speedometer reading. Tire height, differential ratio, number of driving teeth on the transmission. You will need to find a site with a calculator to help you figure out how many teeth you need on your driven gear.


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## Mike O’Neill (Feb 5, 2019)

histoy said:


> My '66 auto speedometer reads 5 mph higher than the actual speed. Is there a general rule about changing the number of teeth on the gear? In other words, if I increase the gear by 1 tooth, how many mph will that change the speedometer reading? On other cars I've worked on it was around 3 mph.


At what actual speed does it read 5 mph high? It should be a constant percentage - if 5 mph high at 50 mph actual, that would be 10% fast, for example.

If you know the percentage, and the actual number of drive teeth and driven teeth, it should be a simple calculation what you need. It may be necessary to change both the drive gear and driven gear.

My Turbo 400 has a pressed-on steel drive gear. I didn't feel like pulling off the transmission tail housing and trying to pull that gear off, so I opted for a ratio adapter:
https://transmissioncenter.net/shop/gm-dodge-ford-ratio-adapter/
Click under Slow down rate to select the percentage correction needed. My speedometer now reads right on the money with my GPS.


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## ralph7 (Aug 3, 2019)

Mike O’Neill said:


> At what actual speed does it read 5 mph high? It should be a constant percentage - if 5 mph high at 50 mph actual, that would be 10% fast, for example.
> 
> If you know the percentage, and the actual number of drive teeth and driven teeth, it should be a simple calculation what you need. It may be necessary to change both the drive gear and driven gear.
> 
> ...


I was quoted about twice the price shown in your link at a nearby speedometer place.
Thanks for the info!!


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## histoy (May 22, 2019)

Mike O’Neill said:


> At what actual speed does it read 5 mph high? It should be a constant percentage - if 5 mph high at 50 mph actual, that would be 10% fast, for example.
> 
> If you know the percentage, and the actual number of drive teeth and driven teeth, it should be a simple calculation what you need. It may be necessary to change both the drive gear and driven gear.
> 
> ...


I read 40 at 35, or 14% high. Given that % when I want to drive 65 on the highway, my speedometer should read 74 mph. I've been driving at 70 to be safe so that's probably why all the other cars are passing me.


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