RPM to MPH Calculator

The RPM to MPH Calculator estimates speed based on your RPM, wheel diameter, and gear ratio. This tool helps automotive enthusiasts and mechanics convert rotational engine data into linear speed. Whether you are calibrating a speedometer, tuning a bicycle, or checking gear ratios, this calculator provides instant results for accurate vehicle analysis.

Enter the rotation speed in revolutions per minute
Enter the wheel diameter in inches
Enter gear ratio if calculating wheel speed from engine RPM (default: 1)

How Speed Is Calculated

Speed represents the distance traveled over a specific period of time. To find this value, we determine the distance the wheel covers in one single rotation.

MPH = (RPM ÷ Gear Ratio) × Wheel Diameter (inches) × π × 60 ÷ 63,360

Where:

  • RPM is the engine revolutions per minute
  • Gear Ratio adjusts for engine-to-wheel rotation differences
  • Wheel Diameter is the size of the tire in inches
  • π (Pi) is approximately 3.14159
  • 60 converts minutes to hours
  • 63,360 converts inches to miles

The process first adjusts the engine RPM by the gear ratio to find the wheel's actual rotation speed. Next, it calculates the tire's circumference using the diameter. Finally, it multiplies the circumference by the wheel rotations to find inches per minute, which is then converted to miles per hour. This standard engineering method ensures precise speed estimates for any vehicle setup.

What Your Speed Means

The result shows your actual linear velocity, helping you understand the relationship between engine work and movement. This number is essential for verifying if your current gear setup is efficient for your desired speed.

Highway Cruising

If your calculation shows 65-75 MPH, your engine is likely running efficiently for long-distance travel. This range is ideal for maximizing fuel economy on open roads.

City Driving

Results between 25-35 MPH indicate typical urban speeds where fuel economy often fluctuates. Use this data to optimize your transmission for stop-and-go traffic conditions.

Bicycle Touring

For cyclists, a result of 10-15 MPH suggests a moderate pace suitable for endurance training. Adjusting your gear ratio can help maintain this speed comfortably over long rides.

Pro Tip

Compare your calculated speed with your dashboard speedometer. If the numbers differ significantly, you may need to recalibrate your sensor or check for tire size differences.

Disclaimer: This calculator provides theoretical estimates based on ideal conditions. Actual speed may vary due to tire wear, road conditions, and transmission efficiency. Use these figures for estimation and planning purposes only.

To ensure the best accuracy, verify your tire measurements by checking the sidewall information rather than relying on rough estimates.

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