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#1 |
Senior Member
![]() Join Date: Mar 2012
Location: ripley ny
Posts: 2,981
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speedometer calibration
i have a 69 gmc k2500 with t-221 and sm 465. i swapped out the 4:56 gear and put in 3:54, and has 33'' tires. anyone know the reducer adapter i may need ? or where to get help?
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#2 |
Registered User
Join Date: Sep 2010
Location: Denver, Colorado
Posts: 1
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Re: speedometer calibration
I like the chart at this link. https://speedometercablesusa.com/gear_box_adapters.html
I have no personal experience correcting speedometer though (yet) |
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#3 | |
Senior Member
![]() Join Date: Mar 2012
Location: ripley ny
Posts: 2,981
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Re: speedometer calibration
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#4 |
Senior Member
![]() Join Date: Mar 2012
Location: ripley ny
Posts: 2,981
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Re: speedometer calibration
thanx for the post, i just found that link u posted, good info. i might have to wait till truck is on the road to do a test then go from there. i found lots of info out there on the subject
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#5 | |
Registered User
Join Date: Jan 2017
Location: Le Sueur MN
Posts: 209
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Re: speedometer calibration
From hatzie PDF of Square Schematics & GM Training Manuals thread.
Quote:
If you want I can do the math for the ratio adapter. I just will need the speedometer gear tooth count. Looks speedometer gear like the picture below. ![]()
__________________
AJC |
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#6 |
Senior Member
![]() Join Date: Mar 2012
Location: ripley ny
Posts: 2,981
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Re: speedometer calibration
im assuming my rockwell has the 4tooth inside and i have what is pictured with 11 teeth, and some very useful info you shared, let me know what you come up with thank you!
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#7 |
Registered User
Join Date: Jan 2017
Location: Le Sueur MN
Posts: 209
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Re: speedometer calibration
You will need a correcting adapter with a ratio of 1.281779661 or as close as possible. Now if your 33" tires run +/-0.5 of an inch, you could still see up to a 3% error. Which would be +/- 2 MPH at 65 MPH.
Here is a GoogleDoc I made that you can play around with tire size and gearing. The formula is from Speedometer Cables USA under "Need a correcting adapter". CORRECTING_ADAPTER_CALCULATOR
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AJC |
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#8 | |
Senior Member
![]() Join Date: Mar 2012
Location: ripley ny
Posts: 2,981
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Re: speedometer calibration
Quote:
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#9 |
Senior Member
![]() Join Date: Mar 2012
Location: ripley ny
Posts: 2,981
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Re: speedometer calibration
speedometer cables hooked me up, gave them my info 1.2844. is the closest i can get
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