1. What is the RPM Formula?

Definition: This calculator estimates engine RPM (revolutions per minute) based on vehicle speed, gear ratio, and tire diameter.

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

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( RPM \) — Engine revolutions per minute
• \( S \) — Vehicle speed in miles per hour (mph)
• \( GR \) — Gear ratio (unitless)
• \( TD \) — Tire diameter in inches

Explanation: The constant 336 combines unit conversions (5280 feet/mile, 12 inches/foot, 60 minutes/hour) divided by π (3.1416).

3. Importance of RPM Calculation

Details: Knowing engine RPM helps optimize performance, fuel efficiency, and prevent engine damage from over-revving.

4. Using the Calculator

Tips: Enter vehicle speed in mph, current gear ratio, and measured tire diameter in inches. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: Where does the 336 constant come from?
A: It's derived from (5280 ft/mile × 12 in/ft) ÷ (60 min/hour × π) ≈ 336.135, rounded to 336.

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

Q3: Does this work for any transmission type?
A: Yes, as long as you use the correct gear ratio for the current gear.

Q4: What's a typical gear ratio?
A: Final drive ratios range from 2.73 to 4.10 for most passenger vehicles.

Q5: Why is RPM important?
A: It helps determine optimal shift points, diagnose drivetrain issues, and select proper gear ratios.