RPM Formulas
A complete reference of every formula used in our calculators, with explanations and units.
Automotive RPM Formulas
⚙️ RPM from Vehicle Speed
Variables: Speed in mph, overall gear ratio (transmission × axle), tire diameter in inches. The constant 336 converts units: (5280 ft/mile × 12 in/ft) ÷ (π × 60 min/hr) ≈ 336.
🚗 Speed from RPM
To convert to km/h: multiply mph by 1.60934. This is simply the inverse of the RPM formula, rearranged to solve for speed.
⚡ Gear Ratio
A ratio greater than 1:1 means the driven gear spins slower (torque multiplication). Less than 1:1 means it spins faster (overdrive).
🔩 Tire Diameter from Size Code
For a 225/65R17 tire: Sidewall = 225 × 0.65 = 146.25 mm; Diameter = (146.25 × 2 / 25.4) + 17 = 28.52 inches.
🔧 Pulley/Belt Output RPM
Belt speed remains constant across pulleys. Smaller driven pulley = higher output RPM (speed increase). Larger driven pulley = lower output RPM (torque increase).
📐 Tire Circumference & Rev/Mile
63,360 = number of inches in one mile (5280 ft × 12 in). Revolutions per mile is useful for speedometer calibration.
Unit Conversions
| From | To | Multiply By |
|---|---|---|
| mph | km/h | × 1.60934 |
| km/h | mph | × 0.62137 |
| inches | millimeters | × 25.4 |
| millimeters | inches | ÷ 25.4 |
| feet/min | meters/sec | × 0.00508 |
| RPM | Radians/sec | × (2π / 60) ≈ × 0.10472 |
| RPM | Hz (rev/sec) | ÷ 60 |
The Constant 336 Explained
Many RPM formulas use the magic number 336. Here's where it comes from:
It combines the unit conversion from miles to inches with the time conversion from hours to minutes, divided by π (to relate diameter to circumference). When you plug in diameter in inches and speed in mph, the units cancel cleanly to give RPM.
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