Gear Ratio & RPM Calculator
Quickly estimate speed at RPM (or RPM at speed) using tire diameter, transmission ratio, and diff ratio. Great for swap planning and gearing decisions.
Speed table
RPM steps up to your selected RPM
| RPM | MPH |
|---|
Understanding the gearing result
Practical context
The road-speed relationship is RPM × tire diameter ÷ (gear ratio × differential ratio × 336) for the common inch/mph approximation used here. A lower numerical gear ratio or taller tire increases road speed per engine revolution; a higher numerical differential or shorter tire increases RPM at a given road speed.
Use actual tire diameter when possible. Nominal tire labels can differ from measured diameter, and worn tires are smaller than new ones. If you are comparing transmissions, enter each individual gear ratio rather than assuming fifth or sixth gear is 1:1.
Gearing changes acceleration feel and traction as well as cruise RPM. A shorter differential multiplies wheel torque but also reaches redline sooner. For track planning, compare the speed at the top of each gear with corner/straight requirements and avoid shifts at inconvenient points.