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#1 |
Registered User
Join Date: Aug 2009
Location: Ft.Collins, Co.
Posts: 101
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Speedo Gear ?
I have a 71 C10 with a 350 / 350 combo . It currently has a 3:08 rear end gear and I am switching to 3:40 gears . What speedo gears do I need to read the correct MPH ? Tires are 255 70 15 .
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#2 |
Senior Member
![]() ![]() Join Date: Jan 2005
Location: CA
Posts: 5,465
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Re: Speedo Gear ?
Is your driven gear housing marked 30-39 or 40-45? Hopefully it's 40-45 because that's what you will need for one of these gear sets:
driving driven 17 red 40 black - will read slightly fast 18 blue 43 purple - will read slightly slow https://www.tciauto.com/speedometer-gear-calculator |
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#3 |
Registered User
Join Date: Aug 2009
Location: Ft.Collins, Co.
Posts: 101
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Re: Speedo Gear ?
Thanks for the advice , I will have to check and see what housing I have .
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#4 |
Senior Member
![]() Join Date: Mar 2015
Location: washington
Posts: 2,233
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Re: Speedo Gear ?
The early 350 had the bullet style speedo gears, didn't they?
9.876% difference between the two gear ratios. The sidewall of the tire is 7" (179 mm), and the tire's circumference is 91.4" (2320.5 mm). This means a 255/70R15 tire will make make 693 revolutions per mile/ If you have the old style bullet gears it will take a 8T and a 19T gear. The 17T and 40T is a ratio of 2.35:1 (new style gears) 8T and a 19T is a ratio of 2.38 (old style gears) So both are with in the allowable speedometer error. |
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#5 |
Registered User
Join Date: Apr 2022
Location: Moorpark, CA
Posts: 755
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Re: Speedo Gear ?
you need to know how many teeth the drive gear is. My TH350 has an 8 tooth drive and the bullet style speedo gear housing.
This calculator is helpful. http://www.stl-vettes.com/65Vette/GearCalc.htm |
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