Vehicle Speed from RPM and Tire Size Calculator

When you change gears, swap tires, or re-gear your axle, knowing your actual road speed requires more than a glance at the speedometer. 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 get your Vehicle Speed in MPH. Secondary outputs include Wheel RPM and Total Gear Ratio so you can see exactly how your drivetrain multiplies engine speed down to the wheels.

RPM

Engine revolutions per minute

inches

Overall tire diameter in inches

:1

Current transmission gear ratio

:1

Ring and pinion gear ratio

Results

Vehicle Speed

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Wheel RPM

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Total Gear Ratio

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Frequently Asked Questions

What is RPM and how does it relate to vehicle speed?

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.

How do I calculate vehicle speed from RPM?

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.

What tire diameter should I use for the calculation?

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.

How do gear ratios affect vehicle speed?

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.

What is the difference between transmission ratio and differential 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.

How accurate is this RPM to speed calculator?

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.

Can I use this calculator for motorcycles or trucks?

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.