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Old 01-27-2007, 04:39 PM
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TH350 Speedometer Gear Choices

Ok, current setup, 26" tall tire, 3.55's, TH350. My drive gear has 7 teeth. The driven gear has 21 teeth. The speedometer reads way faster than what you're really going.

I figured that I need a driven gear with more teeth, but the calculations come in wierd. It's totally backwards of what I was thinking it should be.... it seems that this would actually make it read faster.

Could somebody get me up to speed on this? Here's what the calculations come to be.

Input Parameters Are the Following:

* Speedometer Revolutions/mile = 1001
* Rear Tire Diameter (Inches) = 26.00
* Differential Gear Ratio = 3.55
* Drive Gear Teeth Number = 7

Computation Results:
# Computed Driven Gear is 19
# Ratio of Driven Over Drive Gear is 2.7


Thanks!

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Old 01-27-2007, 04:48 PM
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Taken from a link in the Hotrodders.com knowledge base:

http://chevelleengineer.home.comcast.net/speedo.htm
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Old 01-27-2007, 04:58 PM
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Yeah, ironically, that's a page I came across earlier when trying to think this out. Guess I didn't read it though, because it confirms my thoughts on how it increases/decreases the speed. I think maybe I counted the teeth on the driven gear wrong. I bet that's what it is. I'll be back with the answer after I get back from the parts store.

Thanks, Man!
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Old 01-27-2007, 06:01 PM
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let us know what works, i have the same problem
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Old 01-27-2007, 08:25 PM
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Ok... It was a obvious case of user error, lol. I was indeed counting the teeth wrong, and instead of having 6 drive teeth and 21 driven teeth, it was 10 drive teeth and 21 driven teeth.

I counted the driven gear just fine, looking at it from the end, counting each tooth. Well, I must of not thought about it very much, cause I just counted the number of times around the drive gears teeth were (looking at the face of the gear). Must of been the worm pattern of the teeth that threw me off. But, hey, each tooth twist around the gear 6 times on a 10 tooth gear. Never know, that might be good information to have someday.

Looks like I'll be stepping up to a 28 tooth driven gear.

I think I'm going to order two driven gears, in case I decide to go to 28's (26 tooth driven).

Oh yeah, and by the way, that spring clip broke that holds the driven gear on...so I will need another one of those. That buger does not look like it should be held in so tight after you pres the end of the clip down so it doesn't hook over the end as you slide it back.
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Old 01-27-2007, 08:32 PM
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Ok, lol, apparently 28 tooth is way too big for that drive tooth. Guess I'm going to have to find a matching pair that will make this work.
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Old 01-27-2007, 08:40 PM
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Ok, so, if I just buy a 8 tooth drive gear, and use my existing 21 tooth driven gear in place... everything should work perfectly, right? In fact, if that is the right thing to do, switching between 26" and 28" tires shouldn't make enough of a difference to it for me to need to change a gear. It's like it has a happy medium.

Input Parameters Are the Following:

* Speedometer Revolutions/mile = 1001
* Rear Tire Diameter (Inches) = 26.00
* Differential Gear Ratio = 3.55
* Driven Gear Teeth Number = 21

Computation Results:
# Computed Drive Gear is 8
# Ratio of Driven Over Drive Gear is 2.6



Input Parameters Are the Following:

* Speedometer Revolutions/mile = 1001
* Rear Tire Diameter (Inches) = 28.00
* Differential Gear Ratio = 3.55
* Driven Gear Teeth Number = 21

Computation Results:
# Computed Drive Gear is 8
# Ratio of Driven Over Drive Gear is 2.6
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Old 01-28-2007, 01:26 PM
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Quote:
Originally Posted by skeeveman
Ok, lol, apparently 28 tooth is way too big for that drive tooth. Guess I'm going to have to find a matching pair that will make this work.
try a 22 tooth driven gear i had the same setup or close to it, i had 25.6 tires and 3.42 gear not far off from what u have
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Old 02-02-2007, 12:37 AM
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http://novaresource.org/speedo.htm has what you need. Find the ratio you need, then find the gear set that works for you.
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