Gear Ratio & RPM Calculator
Engine RPM equals speed in mph times the axle ratio times the transmission gear ratio times 336.13, divided by tire diameter in inches. For example, 65 mph with 3.73 gears, a 0.67 overdrive and 30-inch tires turns about 1,820 RPM. Enter your numbers to see RPM and compare two axle ratios.
Results are general information, not mechanic advice. Have a qualified mechanic or tire shop check fitment, speedometer calibration and gearing before you change parts.
FAQ
What is the formula for engine RPM?
RPM = MPH 脳 axle ratio 脳 transmission gear ratio 脳 336.13 梅 tire diameter in inches. 336.13 converts miles per hour and inches into revolutions per minute.
Is a higher or lower axle ratio better for towing?
A numerically higher ratio (like 4.10 vs 3.55) multiplies torque for towing and acceleration, but raises highway RPM and usually lowers fuel economy.
How do bigger tires affect gearing?
Taller tires lower effective gearing, which drops RPM at a given speed and reduces acceleration. Many people re-gear after a big tire upgrade to restore the original feel.
What is a typical overdrive ratio?
Most modern automatics have top gears between about 0.60 and 0.85. Check your owner's manual or transmission spec sheet for exact ratios.