1. What is the MPH Formula Using RPM and Gear Ratio?

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

Purpose: It helps automotive enthusiasts, mechanics, and engineers determine vehicle speed for performance tuning and gear selection.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( S \) — Vehicle speed (miles per hour)
• \( RPM \) — Engine revolutions per minute
• \( TD \) — Tire diameter (inches)
• \( GR \) — Gear ratio (unitless)
• 336 — Constant that combines unit conversions

Explanation: The formula calculates how far the vehicle travels per minute based on wheel rotations, then converts to miles per hour.

3. Importance of Speed Calculation

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

4. Using the Calculator

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

5. Frequently Asked Questions (FAQ)

Q1: Where does the 336 constant come from?
A: It combines unit conversions (5280 feet/mile × 12 inches/foot ÷ 60 minutes/hour ≈ 1056, then 1056 ÷ π ≈ 336).

Q2: How do I measure tire diameter accurately?
A: Measure from ground to top of tire with proper inflation and vehicle weight on tires, or use manufacturer specs.

Q3: What gear ratio should I use?
A: Use your final drive ratio multiplied by transmission gear ratio (e.g., 3.73 rear end × 1.0 in high gear = 3.73).

Q4: Why doesn't my calculated speed match my speedometer?
A: Differences may come from tire wear, incorrect gear ratio input, or speedometer calibration issues.

Q5: Does this work for any vehicle?
A: Yes, as long as you have accurate RPM, tire diameter, and gear ratio values for the specific vehicle.