Gear Ratio Speed Calculator

About this calculator

A vehicle's road speed at a given engine RPM depends on three multiplying factors: the selected gear ratio, the final drive (differential) ratio, and the tire diameter, which together determine how many times the wheels rotate per engine revolution and how much ground each wheel rotation covers. The formula is Speed = (RPM × tire circumference × 60) / (gear ratio × final drive ratio × 63360) for mph (with unit constants adjusted for km/h), since tire circumference converts wheel rotations directly into distance traveled.

Drag racers and circle track racers tuning gear selection, off-road and 4x4 builders choosing tire and axle gear combinations, and automotive engineers and enthusiasts use this calculation to predict top speed in each gear, check whether an engine will stay in its powerband at redline, or verify that a tire size change won't throw off the speedometer.

This calculator takes your engine RPM, gear ratio, final drive ratio, and tire diameter, then returns the resulting road speed in both mph and km/h — much faster than working through the ratio math by hand for every gear and RPM combination.