Calculate number of teeth on speedometer driven gear for 1973 Mustang

MachMan73

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Jan 29, 2020
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I have a 1973 Mustang with a C6 transmission, 245/60R15 rear tires and 3.25 differential. How do I calculate the correct number of teeth for the driven gear? I have seen one source that says the C6 transmission has 6 teeth on the drive gear but cannot confirm this. How many teeth do I need on the driven gear and what is the formula. One equation reference calculates 21 but other equation references calculate 15. The original gear has 18 teeth. It seems to me that I need more teeth.
 

2Blue2

will be trying this sex one when I can find it
Mod Dude
Mar 5, 2019
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Welcome aboard MachMan73

When I do the math, I get 15 also. Thats assuming the 6 tooth drive gear. 7 tooth calculation gives the 18 tooth driven.
I also assume problem is speedometer is reading to fast?
Driven gears are cheap at least, but higher driven tooth counts wear out exponentially faster.
Is the 18 you got in perfect shape?

Post a pic of your stang!
 

MachMan73

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Jan 29, 2020
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Thank you for your input. Someone suggested that I check the actual speed at 10 mph intervals to indicated speed by use of a GPS speedometer app, write down the correlations and calculate the ratio for each 10 mph increment. Then add the ratios together and divide by the number of ratios obtained for the average ratio. Then multiply the number of driven teeth by the ratio and it will provide the number of driven teeth required. I did this for increments from 20 to 70 mph and obtained an average ratio of 1.167. I then multiplied the number of existing teeth (18 x 1.167) and got 21 teeth. I changed from an 18 to 21 tooth driven gear and it now matches spot on with the GPS app. In this manner you do not need to know what rear gear ratio you have or tire diameter. However, knowing my rear gear ratio and tire diameter, I checked several published equations, and found two equations also yielded 21 teeth when entering the rear gear ratio and tire diameter. Questions answered and issue resolved. Thank you for responding.
 

MachMan73

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Jan 29, 2020
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