Gear Ratio Speed Calculator
Calculate the speed of a vehicle based on gear ratio, tire diameter, and engine RPM. Useful for automotive enthusiasts, mechanics, and engineers.
Published by:
isCalculator Editorial Team
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How to use this tool?
This calculator estimates vehicle speed (in mph) based on engine RPM, gear ratio, tire diameter, and final drive ratio.
- Enter Engine RPM – Input your engine’s revolutions per minute (e.g., 3000). Must be a positive number.
- Enter Gear Ratio – The transmission gear ratio (e.g., 3.73). Must be greater than 0.
- Enter Tire Diameter (inches) – The diameter of your tires in inches (e.g., 28). This calculator uses imperial units only.
- Enter Final Drive Ratio – The differential or final drive ratio (e.g., 1). Must be positive.
- Click “Calculate Speed” – The result appears below showing speed in mph, along with the formula used.
Formula: Speed (mph) = (RPM × Tire Diameter) ÷ (Gear Ratio × Final Drive × 336)
All inputs are validated in real time. If any field shows a red error, correct it before calculating.
Previous Results
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Frequently Asked Questions
The formula is: Speed (mph) = (Engine RPM × Tire Diameter in inches) / (Gear Ratio × Final Drive Ratio × 336). The constant 336 converts inches per minute to miles per hour.
Engine RPM is in revolutions per minute, tire diameter is in inches, and the result is in miles per hour (mph). Gear ratios and final drive ratios are unitless.
Yes, as long as you have the correct values for RPM, gear ratio, tire diameter, and final drive ratio. It works for cars, motorcycles, and other wheeled vehicles.
All inputs must be positive numbers. If you enter zero or a negative number, the calculator will display an error and ask you to correct the input.