Gearbox ratio is a ratio of the direct measure of rotational speeds of two or more interlocking gears. The gear ratio is also referred as speed ratio. The gear ratio is determined by the total number of teeth present in two gear. The total of numbers is usually proportional to the radius of gear pitch circle. Use our online gearbox ratio calculator to calculate the gear ratio by choosing transmission and entering the engine RPM, axle ratio, tire height and ratio of each gear.