1. What is a Speed Calculator (MPH from RPM)?

Definition: This calculator estimates vehicle speed in miles per hour based on engine RPM, tire diameter, gear ratio, and final drive ratio.

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

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( \text{MPH} \) — Speed in miles per hour
• \( \text{RPM} \) — Engine revolutions per minute
• \( \text{TD} \) — Tire diameter in inches
• \( \text{GR} \) — Transmission gear ratio
• \( \text{FD} \) — Final drive (differential) ratio
• \( 336 \) — Constant for unit conversion

Explanation: The formula converts rotational speed to linear speed while accounting for gear reduction and tire size.

3. Importance of Speed Calculation

Details: Accurate speed calculation helps in gear selection, fuel efficiency optimization, and ensuring speedometer accuracy.

4. Using the Calculator

Tips: Enter engine RPM, tire diameter (measured from ground to top of tire), current gear ratio (1:1 for direct drive), and final drive ratio. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What's the 336 constant in the formula?
A: It combines unit conversions (inches to miles, minutes to hours) and π (pi) for circumference calculation.

Q2: How do I measure tire diameter accurately?
A: Measure from ground to top of tire with vehicle weight on it, or calculate from tire size markings (e.g., 225/45R17).

Q3: What if I'm in neutral or clutch is disengaged?
A: The calculation assumes power is being transmitted to the wheels through the drivetrain.

Q4: Does this account for tire slippage?
A: No, this calculates theoretical speed. Actual speed may vary due to tire slip, wear, or inflation.

Q5: Can I use this for different units (km/h)?
A: The calculator gives MPH. For km/h, multiply result by 1.60934 or use a tire diameter in centimeters with a different constant.