About this tool
Gear Ratio, RPM & Speed Calculator is a free, in-browser tool that ties together engine speed, road speed, transmission gear ratio, axle (final drive) ratio and tire diameter. Pick which of the five you want to solve for and enter the other four — that field disappears and the result updates live. Everything runs locally in your browser with nothing uploaded.
The relationship is RPM = (MPH × gear ratio × axle ratio × 336.13) ÷ tire diameter, where tire diameter is the overall rolling diameter in inches. The constant 336.13 is 1056 ÷ π, which converts miles per hour and inches into revolutions per minute. Each of the other unknowns is found by rearranging that single equation, so the four inputs fully determine the fifth.
Use it to predict cruising RPM after an axle-ratio or tire-size change, to pick a final drive for a target highway RPM, or to check a speedometer reading against engine RPM. In top gear the transmission ratio is usually 1.0 (or the listed overdrive ratio).