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#1 |
Chance
![]() Join Date: Sep 2001
Location: Belcourt ,ND, USA
Posts: 750
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Speedo Gear Tooth Difference
I am still fighting with my truck. Now I am working on getting the speedometer accurate. I have a 45 Tooth Driven gear and at 65mph the speedometer in my truck reads 53mpg. I put another driven gear in (didn't count it, but I assume it had less teeth) and at 65mph the speedometer was reading 57mph. I have heard that for every tooth difference there is a 5mph change in speed at the speedometer. Is this theory right? My Fiance` was nice enough to help me out with the acutal speed. Please help a guy out. Thank!!
Chance
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1968 Chevrolet Stepside BB 396 w/TH400 1972 Chevrolet 3/4 Ton 4x4 BB 496 w/TH400 |
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#2 |
Registered User
Join Date: Oct 2003
Location: San Jose Ca.
Posts: 3,334
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Re: Speedo Gear Tooth Difference
There are 2 gears that you have to work with. The speedo gear and the gear inside the tranny. What you can do is this. Find out what gear is inside the tranny and go from there. Also here are some links that can help you out. And a thread from another guy that went through the same thing you did.
http://67-72chevytrucks.com/vboard/s...d.php?t=204866 http://www.bgsoflex.com/speedo1.html http://www.tciauto.com/components/speedometer_gears.htm http://www.transmissioncenter.net/sp...n_______va.htm
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RIP to my son Dylan. 5/17/2002 - 8/18/2018 If you want to know more about rare forms of pediatric cancer or if you want to help fund pediatric cancer research, please check out the website for the non-profit I started. https://slothforpedcancer.org 68 build thread http://67-72chevytrucks.com/vboard/s...d.php?t=223234 |
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#3 |
Registered User
Join Date: Sep 2005
Location: 9000' Mountain Man
Posts: 326
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Re: Speedo Gear Tooth Difference
There are speedometer adapters available that mount between the transmission and the speedometer cable. They have three small gears inside. Many pickups and trucks came from the factory with these adapters. I know speedometer shops still offer them and all you have to do is tell them what ratio you need. The last time I checked, they had a price tag of about $75.00 on them. I think they are a little pricy for what you get but it's a quick and easy way to correct the speedometer.
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#4 |
Senior Member
![]() Join Date: Jul 2004
Location: Estherville, Iowa
Posts: 3,371
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Re: Speedo Gear Tooth Difference
You can go as small as 34 teeth, but the 34-39 require a different driven gear housing, the housing you have supports 40-45 teeth. If the 34 tooth isn't enough then you have to change the driven gear to a larger tooth count.
Best to find a online calculator, you can get it right by trial and error, but it is better and cheaperto get the tooth count right the first time if possible.
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1968 C10 307 3spd Long Fleet ------ http://67-72chevytrucks.com/vboard/s...d.php?t=201103 1970 C10 305 Super T10 Long Fleet --- http://67-72chevytrucks.com/vboard/s...d.php?t=202285 1971 C20 383 TH350 Dana Posi ----- http://67-72chevytrucks.com/vboard/s...d.php?t=206894 2001 GMC Sierra 1500 C3 6.0 |
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#5 |
Chance
![]() Join Date: Sep 2001
Location: Belcourt ,ND, USA
Posts: 750
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Re: Speedo Gear Tooth Difference
Here's what I have at an actual speed of 65mph...
With a 45T driven gear the odometer read 53mph. With a 42T driven gear the odometer read 57mph. I am thinking I need a 38T gear that requires another housing. I call the transmission shop where I purchased it from with my assumption and they agreed. It was also nice that they are gonna mail me what I need. So hopefully they come through.
__________________
1968 Chevrolet Stepside BB 396 w/TH400 1972 Chevrolet 3/4 Ton 4x4 BB 496 w/TH400 |
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#6 |
Aluminum slot wheels rule...
Join Date: Jan 2006
Location: Maine
Posts: 6,212
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Re: Speedo Gear Tooth Difference
I wish my truck could go 65mph.
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