OK, C3 Speedo experts
I have a very early 69' 350/350 with what I now believe to be an M20 trans and 3.08 rear end. The speedo was registering something like 20mph lower than actual speed. I have reviewed many other post regarding the speedo gears and found the following calculation.
If you want to do the math yourself for the exact tires you are running, it is not hard to do. The speedometer is calibrated for the speedometer cable to turn 1000 times per mile.
To do the calculation, start by measuring the circumference of your rear tire, in inches, with a tape measure. Divide the number of inches in a mile (5280x12=63,360) by the circumference of your tire to get the number of revolutions your tire will turn per mile. The answer will be around 740 revolutions per mile for a typical size C2 tire.
Now multiply by your rear axle ratio (3.70 or 3.73 ??) to get the driveshat revolutions-per-mile. The answer will be around 2750. Multiply this by the quotient of the DRIVE gear tooth count over the DRIVEN gear tooth count, such as (8/22 = .364). A driveshaft speed of 2700 revolutions-per-mile will be scaled by this factor to yield (2750 x 0.364)= 1001 revolutions-per-mile (remember, the target is 1000).
If your calculations show that a 22 tooth gear is not quite right for your tires, I believe that the other DRIVEN gears available for the 8-tooth DRIVE gear include 21 teeth and 24 teeth.
This calcualtion is for a tire with a circumfrnce of 85 inches, which I have. So, 740 x 3.08 = 2279, then 2279 * (8/18 or .44) = 1002.848
Is this calculation correct? Can anyone verify this? I do not know which speedo gear I had becasue it is gone. I just ordered an 18, 19 and 20 tooth set of gears to try and fix this. From the above it looks like the 18 tooth is correct unless the speedo was way out of calibration.
Thoughts anyone? Thanks for the help you guys are great.
Joe
If you want to do the math yourself for the exact tires you are running, it is not hard to do. The speedometer is calibrated for the speedometer cable to turn 1000 times per mile.
To do the calculation, start by measuring the circumference of your rear tire, in inches, with a tape measure. Divide the number of inches in a mile (5280x12=63,360) by the circumference of your tire to get the number of revolutions your tire will turn per mile. The answer will be around 740 revolutions per mile for a typical size C2 tire.
Now multiply by your rear axle ratio (3.70 or 3.73 ??) to get the driveshat revolutions-per-mile. The answer will be around 2750. Multiply this by the quotient of the DRIVE gear tooth count over the DRIVEN gear tooth count, such as (8/22 = .364). A driveshaft speed of 2700 revolutions-per-mile will be scaled by this factor to yield (2750 x 0.364)= 1001 revolutions-per-mile (remember, the target is 1000).
If your calculations show that a 22 tooth gear is not quite right for your tires, I believe that the other DRIVEN gears available for the 8-tooth DRIVE gear include 21 teeth and 24 teeth.
This calcualtion is for a tire with a circumfrnce of 85 inches, which I have. So, 740 x 3.08 = 2279, then 2279 * (8/18 or .44) = 1002.848
Is this calculation correct? Can anyone verify this? I do not know which speedo gear I had becasue it is gone. I just ordered an 18, 19 and 20 tooth set of gears to try and fix this. From the above it looks like the 18 tooth is correct unless the speedo was way out of calibration.
Thoughts anyone? Thanks for the help you guys are great.
Joe
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