RPM to MPH Calculator

Convert revolutions per minute (RPM) to miles per hour (MPH) by entering the RPM value and wheel diameter. This calculator is useful for determining vehicle speed from engine or wheel rotation.

How to Use This Calculator

Enter the RPM value of your engine or wheel
Input the wheel diameter in your preferred unit
Adjust the gear ratio if you're calculating wheel speed from engine RPM
Click Calculate to see the speed in miles per hour

Formula Used

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

Where:

RPM = Revolutions per minute of the engine or wheel
Gear Ratio = Ratio between engine and wheel rotation
Wheel Diameter = Diameter of the wheel in inches
π = Mathematical constant pi (approximately 3.14159)
60 = Minutes per hour
63,360 = Inches per mile

Example Calculation

Real-World Scenario:

A car is traveling at 3,000 RPM with a wheel diameter of 24 inches and a gear ratio of 4:1. Let's calculate the speed in MPH.

Given:

RPM = 3,000
Wheel Diameter = 24 inches
Gear Ratio = 4

Calculation:

MPH = (3,000 ÷ 4) × 24 × π × 60 ÷ 63,360

MPH = 750 × 24 × π × 60 ÷ 63,360

MPH = 3,392,920 ÷ 63,360

Result: 53.6 MPH

Why This Calculation Matters

Practical Applications

Determining vehicle speed from tachometer readings
Calculating bicycle speed from cadence and gear ratios
Engineering applications for rotating machinery

Key Benefits

Helps select appropriate gear ratios for desired speeds
Useful for speedometer calibration
Assists in performance tuning and optimization

Common Mistakes & Tips

The formula requires the wheel diameter, not the radius. If you only know the radius, multiply it by 2 to get the diameter. Using radius will result in a speed calculation that is half of the actual value.

When calculating speed from engine RPM, the gear ratio is crucial. The engine RPM must be divided by the gear ratio to determine the wheel RPM. Forgetting to account for this will result in significantly overestimated speeds.

Frequently Asked Questions

You can find the wheel diameter by checking the tire size printed on the sidewall. For example, a tire marked "225/45R17" has a diameter of approximately 25 inches. You can also measure directly from the ground to the top of the tire and multiply by 2.

Gear ratios vary significantly between vehicles and gears. In first gear, ratios typically range from 3:1 to 4:1, while in higher gears they might be around 1:1 or even less (overdrive). For calculating highway speed, you would typically use the ratio for the highest gear (often 0.7:1 to 1:1).

Yes, this calculator works well for bicycles. For a bike, you would use your pedaling cadence as the RPM and include the gear ratio from your chainring to cassette. Typical bike wheels have diameters of 26-29 inches, and pedaling cadence is usually 60-100 RPM.

References & Disclaimer

Engineering Disclaimer

This calculator provides theoretical speed calculations based on the input parameters. Actual vehicle speed may vary due to factors such as tire deformation, road conditions, transmission efficiency, and other mechanical variables. This tool should be used for estimation purposes only and not for critical engineering applications without professional verification.

References

Revolutions Per Minute (RPM) - Wikipedia - Explanation of RPM as a unit of rotational speed
Gear Ratio - Wikipedia - Information on gear ratios and their applications
Calculating Tire Dimensions - Tire Rack - Guide to calculating tire dimensions from size markings

Accuracy Notice

This calculator provides results based on the mathematical formula for converting rotational speed to linear speed. Results are accurate to the extent that input values are correct and represent ideal conditions. Real-world applications may have variations due to factors not accounted for in this calculator.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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science physics rpm mph engineering formula

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