RPM Formula:
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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.
The calculator uses the formula:
Where:
Explanation: The formula accounts for the relationship between wheel rotation and engine rotation through the drivetrain.
Details: Knowing engine RPM helps optimize gear selection, maintain engine efficiency, and prevent over-revving that could damage the engine.
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.
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.