Work out bicycle gear inches instantly with clear inputs, formula shown and shareable results.
Gear inches is the ratio times wheel diameter in inches — the equivalent diameter of a direct-drive penny farthing wheel. Development is the distance travelled per pedal revolution, which combined with cadence gives road speed directly.
Gear inches
gear inches = (chainring / cog) x wheel diameter (in); development = ratio x pi x wheel diameter
About 30-40 for a low climbing gear, 70-80 for cruising and 100-120 for a high top gear. Track sprinters use 100-110 fixed.
It is metric and intuitive: 7 m per pedal stroke at 90 rpm is 37.8 km/h. Gear inches is a historical convention that survives in English-speaking cycling.