1. What is the RPM Formula?

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

Purpose: It helps automotive enthusiasts and mechanics understand the relationship between these variables for performance tuning and gear selection.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( RPM \) — Engine revolutions per minute
• \( S \) — Vehicle speed (miles per hour)
• \( GR \) — Gear ratio (unitless)
• \( TD \) — Tire diameter (inches)
• \( 336 \) — Constant to convert units

Explanation: The formula calculates how fast the engine must turn to achieve a certain road speed given the gear reduction and tire size.

3. Importance of RPM Calculation

Details: Proper RPM calculation helps optimize engine performance, fuel efficiency, and transmission gear selection.

4. Using the Calculator

Tips: Enter the vehicle speed in mph, gear ratio (e.g., 3.73), and tire diameter in inches. 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) with the π (pi) calculation for tire circumference.

Q2: How do I measure tire diameter?
A: Measure from ground to top of tire or use manufacturer specifications. For example, a "275/60R15" tire is about 28 inches in diameter.

Q3: What's a typical gear ratio?
A: Common ratios range from 2.73 (highway gears) to 4.10 (performance/towing gears).

Q4: Does this account for torque converter slip?
A: No, automatic transmissions may show slightly higher RPM due to converter slip, especially under load.

Q5: How accurate is this calculation?
A: It's theoretically accurate for manual transmissions. Real-world results may vary slightly due to tire growth, slip, and other factors.