Gear Ratio & RPM to Speed
Calculate vehicle speed at a given RPM for any gear ratio and final drive combination, in both mph and km/h.
Results
What is it?
The gear ratio and RPM to speed calculator tells you exactly how fast your vehicle is moving at a given engine speed in any gear. It accounts for the transmission gear ratio, final drive (differential) ratio, and tire diameter � the complete drivetrain multiplication chain.
How to use
Enter the engine RPM (e.g. peak power RPM), the transmission gear ratio (from your gearbox specification sheet), the final drive ratio (usually printed on the axle or in the service manual), and your tire's overall diameter in inches. Speed is computed in both mph and km/h.
Example scenario
A car at 3,000 RPM in 3rd gear (ratio 3.2:1) with a 3.73 final drive and 26-inch tires: speed = 3000 x 26 x 3.1416 / (3.2 x 3.73 x 336) = 57.8 mph (93 km/h). At redline 7,000 RPM, that same gear gives 134.9 mph � useful for identifying gear changes at a track.
Pro tip
Shorter final drive ratios (higher numbers, e.g. 4.10 vs 3.27) improve acceleration but hurt top speed and fuel economy. Use this calculator to build a full RPM-vs-speed chart for each gear and identify where power band overlap occurs between gears � the optimal shift point is where the higher gear enters the power band.