[C2] Speedometer Problem
Thread Starter
Le Mans Master
Joined: Apr 2013
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From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
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Speedometer Problem
The speedo on my 65 is 5mph too slow. Running a 22 tooth so need either a 21 or 20. But, in my research, they only make a 22, 23, & 24 in a large gear which I need. Why I don’t know, doesn’t make sense. Why would a manufacturer make two different sets of worm gears, a large and a small? And, if I’m correct, Borg-Warner did it and so did Muncie. Is it because of the seemingly endless combinations of axle ratios and tire sizes?
Other than reworking my tranny or speedo, is a ratio adapter my only option, and, if so, where can I get one that fits my car?
Other than reworking my tranny or speedo, is a ratio adapter my only option, and, if so, where can I get one that fits my car?
Last edited by 59BlueSilver; Mar 13, 2018 at 06:39 AM.
Race Director
Joined: Jan 2003
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From: Central Arkansas
It's primarily the diameter change required on the driven gear as the number of teeth changes. As the number of teeth gets less, at some point they have to reduce the gear diameter to maintain tooth width and that requires a different spiral drive gear to compensate. Mopar dealt with this a little differently by using an offset speedometer drive housing that is rotated to different points to vary the distance of the driven gear center, to the spiral drive gear to compensate. Just two different approaches.
I think you may have to go to an adaptor but maybe someone else has another idea short of pulling the transmission tailhousing and replacing the drive gear.
I think you may have to go to an adaptor but maybe someone else has another idea short of pulling the transmission tailhousing and replacing the drive gear.
Le Mans Master
Joined: Nov 2015
Posts: 7,495
Likes: 2,654
From: Really Central IL Illinois
The speedo on my 65 is 5mph too slow. Running a 22 tooth so need either a 21 or 20. But, in my research, they only make a 22, 23, & 24 in a large gear which I need. Why I don’t know, doesn’t make sense. Why would a manufacturer make two different sets of worm gears, a large and a small? And, if I’m correct, Borg-Warner did it and so did Muncie. Is it because of the seemingly endless combinations of axle ratios and tire sizes?
Other than reworking my tranny or speedo, is a ratio adapter my only option, and, if so, where can I get one that fits my car?
Other than reworking my tranny or speedo, is a ratio adapter my only option, and, if so, where can I get one that fits my car?
http://www.stl-vettes.com/65vette/gearcalc.htm
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,001
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
Just in case you have not considered your axle ratio and tire size:
http://www.stl-vettes.com/65vette/gearcalc.htm
http://www.stl-vettes.com/65vette/gearcalc.htm
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,001
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
It's primarily the diameter change required on the driven gear as the number of teeth changes. As the number of teeth gets less, at some point they have to reduce the gear diameter to maintain tooth width and that requires a different spiral drive gear to compensate. Mopar dealt with this a little differently by using an offset speedometer drive housing that is rotated to different points to vary the distance of the driven gear center, to the spiral drive gear to compensate. Just two different approaches.
I think you may have to go to an adaptor but maybe someone else has another idea short of pulling the transmission tailhousing and replacing the drive gear.
I think you may have to go to an adaptor but maybe someone else has another idea short of pulling the transmission tailhousing and replacing the drive gear.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,001
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
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Melting Slicks
Joined: Sep 2005
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From: merritt island fl
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I went all through this with my '56. Anyway, my notes say the GM part number is3860343 for a large 20 tooth. It's blue.
Edit: I did a few quick searches and it looks like it is unobtanium.
Edit: I did a few quick searches and it looks like it is unobtanium.
Last edited by davekp78; Mar 13, 2018 at 11:41 AM.
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Thread Starter
Le Mans Master
Joined: Apr 2013
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From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
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Yes, I agree, that looks like the correct part number I need. Thanks much! Now to find it!
Team Owner
Joined: Jul 2006
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From: Kannapolis NC
https://www.ebay.com/itm/1961-and-Up...FUN~hl&vxp=mtr
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,001
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
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I don't know anything about this company, but it might be worth $13.05 including shipping to try.
https://www.ebay.com/itm/1961-and-Up...FUN~hl&vxp=mtr
https://www.ebay.com/itm/1961-and-Up...FUN~hl&vxp=mtr