1. What is an RPM to MPH Calculator?

Definition: This calculator estimates vehicle speed in miles per hour based on engine RPM and tire diameter.

Purpose: It helps automotive enthusiasts, mechanics, and engineers determine vehicle speed at specific RPMs for gear ratio analysis and performance tuning.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( S \) — Speed (miles per hour)
• \( RPM \) — Engine revolutions per minute
• \( TD \) — Tire diameter (inches)
• 336 — Constant combining unit conversions (5280 ft/mile × 12 in/ft ÷ 60 min/hour ÷ π)

Explanation: The formula calculates how much distance the vehicle travels per minute based on tire circumference and RPM, then converts to miles per hour.

3. Importance of RPM to Speed Calculation

Details: Understanding the relationship between RPM and speed helps with gear selection, fuel efficiency optimization, and performance tuning.

4. Using the Calculator

Tips: Enter the engine RPM and tire diameter in inches. For accurate results, measure your actual tire diameter (not just the sidewall number).

5. Frequently Asked Questions (FAQ)

Q1: Why is the constant 336 used?
A: It combines all the unit conversions (miles to feet, feet to inches, hours to minutes, and circumference calculation) into one constant.

Q2: Does this account for gear ratios?
A: No, this assumes direct 1:1 drive ratio. For real calculations, you'll need to factor in your transmission and differential gear ratios.

Q3: How do I measure my actual tire diameter?
A: Measure from the ground to the top of the tire with the vehicle's weight on it, or calculate using: (wheel diameter) + 2×(sidewall height).

Q4: Why doesn't my actual speed match the calculation?
A: Differences can come from gear ratios, torque converter slip (automatics), tire wear, or incorrect tire diameter measurement.

Q5: Can I use this for different units?
A: This calculator is specifically for MPH. For km/h, you would need to use a different constant (approximately 208).