Speedometer gears - Pontiac GTO Forum
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post #1 of 5 (permalink) Old 07-17-2019, 06:05 PM Thread Starter
 
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Speedometer gears

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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post #2 of 5 (permalink) Old 08-06-2019, 10:49 PM
 
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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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post #3 of 5 (permalink) Old 08-10-2019, 02:59 PM
 
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Originally Posted by histoy View Post
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/...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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1967 Pontiac GTO 400, TH400 transmission
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post #4 of 5 (permalink) Old 08-10-2019, 03:55 PM
 
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Originally Posted by Mike O誰eill View Post
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/...ratio-adapter/
Click under Slow down rate to select the percentage correction needed. My speedometer now reads right on the money with my GPS.
I was quoted about twice the price shown in your link at a nearby speedometer place.
Thanks for the info!!
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post #5 of 5 (permalink) Old 08-10-2019, 08:07 PM Thread Starter
 
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Originally Posted by Mike O誰eill View Post
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/...ratio-adapter/
Click under Slow down rate to select the percentage correction needed. My speedometer now reads right on the money with my GPS.
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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