How to calibrate speedometer?

Discussion in 'General Automotive Tech' started by Paladin62, May 8, 2020.

  1. Paladin62

    Paladin62 Well-Known Member

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    My speedometer is off. I have all the specs, but haven't found the right formula. The tranny is a Turbo 350. the rear end is 336 - the tires are 28" tall with circumference of 87.9 - revs/mile=721. How many teeth should be on the transmission gear?
     
  2. Silvertwinkiehobo

    Silvertwinkiehobo "Everything that breaks starts with 'F.'"

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    You have to have a workable ratio between the output shaft and the cable--only changing one may not get you as close as you like to be. Plus, if I recall, the equation requires both gear teeth numbers. It's on the Net, just google for it.
     
  3. Doghead

    Doghead Well-Known Member

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    Even then, it'll be impossible to get it dead nuts accurate. The only way to stay legal would be to run a second speedo which measures against GPS location
     
  4. HotRodRacer

    HotRodRacer Moderator Staff Member Moderator

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    I had the same issue when I swapped from a three speed manual to a TH350 in my 71 truck. I might have been doing 50 MPH and the speedo said 100 or so.

    Here is the link that I used to calculate what driven gear I needed (the one on the end of the speedo cable). TCI also sells those gears, imagine that. :) I got the gear through Summit Racing.

    https://www.tciauto.com/speedometer-gear-calculator

    You'll have to know what you described above, plus the number of teeth on the drive gear. Easy enough, pull the speedo gear out, shine a light in, and see what color it is. The color codes to number of teeth chart is on the same page as the calculator.

    Hope this helps.
     

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