Gear Ratio & RPM Calculator

How Gear Ratios & RPM Work

Your engine's RPM is determined by how fast the wheels are spinning, multiplied by all the gear reductions between the crankshaft and the tire contact patch. The formula uses two ratios: the transmission's ratio for the selected gear, and the final drive (differential) ratio.

Where Total Ratio = Transmission Gear Ratio × Final Drive Ratio and the constant 336 accounts for unit conversion (miles per hour → inches per minute → revolutions).

Common Gear Ratios by Platform

• BMW ZF 8HP: 1st 4.714 / 2nd 3.143 / 3rd 2.106 / 4th 1.667 / 5th 1.285 / 6th 1.000 / 7th 0.839 / 8th 0.667
• Porsche PDK 7-speed: 1st 3.909 / 2nd 2.471 / 3rd 1.720 / 4th 1.288 / 5th 1.000 / 6th 0.839 / 7th 0.660
• Tremec T56/TR6060: 1st 2.660 / 2nd 1.780 / 3rd 1.300 / 4th 1.000 / 5th 0.740 / 6th 0.500

Effect of Final Drive on Performance

A numerically higher final drive ratio (e.g., 4.10 vs 3.23) multiplies torque to the wheels more aggressively, improving acceleration but raising RPM at cruise speed and hurting fuel economy. Swapping final drive gears is one of the most effective ways to tune the power delivery character of a car.

Tire Size Impact

A taller tire effectively lengthens your gear ratios — your RPM will drop at any given speed, which can make the car feel lazier off the line. A shorter tire has the opposite effect. Always recalculate after a tire size change.