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Calculate Engine Rpm From Gear And Tire Size

RPM Formula:

\[ RPM = \frac{S \times GR \times 336}{TD} \]

mph
inches

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1. What is an Engine RPM Calculator?

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

Purpose: It helps automotive professionals and enthusiasts determine engine speed for gear selection, performance tuning, and fuel efficiency calculations.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ RPM = \frac{S \times GR \times 336}{TD} \]

Where:

Explanation: The formula accounts for the relationship between wheel rotation and engine rotation through the drivetrain.

3. Importance of RPM Calculation

Details: Knowing engine RPM helps optimize gear selection, maintain engine efficiency, and prevent over-revving that could damage the engine.

4. Using the Calculator

Tips: Enter the vehicle speed in mph, gear ratio (e.g., 3.55 for common rear axle ratios), and tire diameter in inches. All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What is the 336 constant in the formula?
A: It combines unit conversions (5280 feet/mile, 12 inches/foot, 60 minutes/hour) divided by π (≈3.1416).

Q2: How do I measure tire diameter?
A: Measure from ground to top of tire (loaded) or use manufacturer specs. For formula: diameter = (width * aspect ratio / 100 * 2 / 25.4) + wheel diameter.

Q3: Does this account for torque converter slip in automatics?
A: No, automatic transmissions may show slightly higher RPM due to converter slip, especially under load.

Q4: What's a typical gear ratio?
A: Final drive ratios range from 2.73 (highway) to 4.10+ (performance). Transmission gear ratios vary by gear.

Q5: Why does RPM matter for fuel economy?
A: Engines have optimal RPM ranges for efficiency. Too high wastes fuel, too low may lug the engine.

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