TRANS-02, Speedometer, Gear Ratio, and Tire Size Manual Calculations

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Use the following formula to determine tire circumference:

Tire Circumference Formula

As an example, for a tire of size 225/50 VR 16:

Tread Width 225 mm
Aspect Ratio 50
Wheel Diameter 16 in.

Aspect Ratio is used to determine the height of the sidewall in terms of percentage of the tread width. In the example given the height of the sidewall is 50% of the tread width or 112.5 mm. In the formula above, we would use .50 for the aspect ratio.

Multiplying by 2 in the formula accounts for the height of the sidewall at the top and bottom of the wheel and dividing by 25.4 converts the total wall height from millimeters to inches. By adding the wheel diameter (given in inches) to the wall height, we obtain total diameter.

Using the formula for circumference of a circle, we obtain the circumference of the tire.

Circumference Formula

Substituting for the tire size in the example:

Example Tire Size Formula

To determine the speed in MPH for a given gear ratio use the following formula:

Speed Formula

So, the calculated speed for a 1983 944 in 5th gear at 6400 RPM with 225/50 VR 16 tires would be:

Example Speed

Clark's Garage 1998