Gear Ratio, RPM & Tire Speed Calculator
Calculate engine RPM at highway speeds, transmission overdrive ratios, differential ring-and-pinion gearing, and tire diameter revs per mile.
Calculated Results
Frequently Asked Questions
What is the formula to calculate engine RPM from gear ratio and tire size?
The classic automotive formula is: RPM = (MPH × Gear Ratio × Axle Ratio × 336) / Tire Diameter in inches. The constant 336 accounts for unit conversions between miles, hours, inches, and minutes.
What happens to RPM when you install larger tires on a truck?
Larger diameter tires act like a taller gear: they lower your cruising RPM at highway speeds, but reduce off-the-line low-end torque acceleration and cause your speedometer to read slower than your actual speed.
What is the difference between a 3.08 and a 4.10 differential axle ratio?
A 4.10 gear is 'shorter' (higher numerically), providing 33% more wheel torque for towing heavy trailers and turning big tires, but causing the engine to turn at higher RPMs. A 3.08 gear is 'taller', reducing engine RPM on the interstate for maximum fuel economy.
What is overdrive in an automotive transmission?
Overdrive is any transmission gear where the output shaft rotates faster than the input shaft (a ratio less than 1.00:1, such as 0.70:1). This allows the engine to maintain low RPMs while sustaining high cruising speeds.