1. What is an RPM From Gear Calculator?

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

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

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 many times the engine must rotate to move the vehicle at a given speed based on gear reduction and tire circumference.

3. Importance of RPM Calculation

Details: Proper RPM calculation ensures optimal engine performance, fuel efficiency, and helps prevent over-revving the engine.

4. Using the Calculator

Tips: Enter the vehicle speed in mph, gear ratio (e.g., 3.73 for common rear axle ratios), and actual 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 ft/mile, 12 in/ft, 60 min/hour) and π to simplify the formula.

Q2: How do I find my gear ratio?
A: Check your vehicle specifications or count driveshaft rotations vs. wheel rotations (in neutral).

Q3: Should I use tire size or actual measured diameter?
A: For accuracy, measure from ground to top of tire under load, as sidewall markings may be inaccurate.

Q4: Does this work for manual and automatic transmissions?
A: Yes, but account for torque converter slip in automatics by adding 5-10% RPM.

Q5: What's a safe RPM range?
A: Most engines perform best between 1500-4000 RPM, but check your specific engine's redline.