Enter your Engine RPM, Tire Diameter, Transmission Gear Ratio, and Differential/Rear Gear Ratio into the Vehicle Speed from RPM and Tire Size Calculator to find your Vehicle Speed, along with your Wheel RPM and Total Gear Ratio — great for dialing in gear swaps or checking how a tire size change affects your top end.
Vehicle Speed
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Wheel RPM
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Total Gear Ratio
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RPM (Revolutions Per Minute) measures how fast your engine's crankshaft rotates. Vehicle speed depends on engine RPM, tire size, and the gear ratios in your transmission and differential, which determine how engine rotation translates to wheel rotation.
Vehicle speed is calculated using the formula: Speed = (RPM × Tire Diameter × π × 60) / (63360 × Transmission Ratio × Differential Ratio). The calculator automatically performs this calculation for accurate results.
Use the overall diameter of your tire, which includes the wheel rim and tire sidewall height. You can find this on tire calculators or measure from ground to the top of the tire. Common diameters range from 24-32 inches for most vehicles.
Higher gear ratios (numerically larger) reduce speed but increase torque multiplication. Lower ratios allow higher speeds at the same RPM. Both transmission and differential ratios work together to determine final drive ratio.
Transmission ratio varies by gear (1st, 2nd, etc.) and ranges from about 0.7:1 to 4:1. Differential ratio is fixed (like 3.73:1) and determines the final drive reduction from the transmission output to the wheels.
The calculator provides theoretical speeds based on mathematical relationships. Real-world factors like tire pressure, drivetrain loss (typically 15-20%), road conditions, and tire wear can affect actual vehicle speed.
Yes, the physics principles apply to all vehicles. Just ensure you use the correct tire diameter and gear ratios for your specific vehicle type. Motorcycles typically have different gear ratio ranges than cars.