Speedometer and Gear Ratio Manual Calculation Formulas
Manual calculation formulas for tire circumference and speed in any gear from engine RPM, gear ratio, final drive ratio, and tire dimensions — with a worked example for a 1983 944.
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These formulas allow you to calculate tire circumference and vehicle speed from engine RPM, gear ratios, final drive ratio, and tire dimensions. A worked example using a 1983 944 is provided at the end.
Tire Circumference Formula
The circumference of a tire cannot be measured directly from the tire specification because the specification mixes millimeters (sidewall height) and inches (wheel diameter). The formula below converts everything to inches before computing the circumference.
Where:
- W = tread width in mm (first number in tire specification)
- A = aspect ratio as a percentage (second number in tire specification)
- D = wheel diameter in inches (third number in tire specification)
- C = tire circumference in inches
This formula is an application of the basic circumference-of-a-circle formula:
The expression inside the brackets builds up the total tire diameter by adding the wheel diameter to twice the sidewall height (one sidewall on each side of the wheel), with the sidewall height converted from millimeters to inches by dividing by 25.4.
Worked Example — Tire Circumference
Using a 225/50 VR 16 tire:
- W = 225 mm (tread width)
- A = 50 (aspect ratio; sidewall height = 50% of 225 mm = 112.5 mm)
- D = 16 in. (wheel diameter)
The circumference of a 225/50 VR 16 tire is approximately 75.5 inches.
Speed Formula
Once tire circumference is known, speed in any gear can be calculated from engine RPM, the gear ratio for that gear, and the final drive ratio.
Where:
- V = vehicle speed in MPH
- RPM = engine RPM
- C = tire circumference in inches (from the formula above)
- FDR = final drive ratio
- GR = gear ratio for the selected gear
- 1056 = unit conversion constant (converts inches per minute to miles per hour)
Worked Example — Speed
Using a 1983 944 in 5th gear at 6400 RPM with 225/50 VR 16 tires:
- C ≈ 75.5 in. (from the circumference example above)
- FDR = 3.889 (1983 944 final drive ratio)
- GR = 0.8286 (1983 944 5th gear ratio)
The theoretical maximum speed of a 1983 944 in 5th gear at 6400 RPM on 225/50 VR 16 tires is approximately 142 MPH.
Note: Gear ratios and final drive ratios for all 924/944/968 transaxle variants are listed in TRANS-04. The interactive version of these calculations is described in TRANS-01.