How to Convert Gear Ratio to MPH

If you know the gear ratio, tire diameter, and RPMs of your car at a given speed, you can easy calculate the exact miles per hour the car is traveling using a simple formula. This formula can be used to double-check how accurate your speedometer is, and whether it may need to be adjusted. It is important to note that a specific gear ratio will tell you the performance speeds you should expect from your car, but will not by itself tell you the miles per hour of the car at a given time, because all three variables must be present to determine the vehicle's exact speed.

Things You'll Need

• Ruler
• Calculator

Instructions

1. Measure the tire diameter of your car with the ruler if you do not know it.

2. Use the RPM meter to determine the revolutions per minute of the car at the given speed.

3. Multiply the rpms by the tire diameter.

4. Multiply the gear ratio by 336.

5. Divide the number obtained in Step 3 by the number obtained in Step 4. The resulting number is the miles per hour of the car.

References

• Offroaders: Tires, Gears, RPMs, and MPH