Gear Inches by lee walker

A gear inch measures how far your bike travels with each pedal stroke.  Riders considering a gearing change, or a bike with different sized wheels can use this data to anticipate the effect these changes will have on the gearing.  If you're riding along at a constant speed and you upshift, your cadence is reduced, because your gear inch has increased.  You are travelling farther with each revolution of the pedals.  This is a result of the ratio between the number of teeth you are spinning around on the chainring and the number of teeth you are turning on the cassette.  A ratio of 53/11 (4.8) is much bigger than a ratio of 39/25 (1.5).  Thus to ride the same speed in each of these gears, your cadence would be MUCH lower with the bigger 53/11 gear and much faster than the smaller 39/25.  Likewise, at a given cadence, each gear will produce a vastly different speeds, since every pedal stroke sends you much farther with the bigger gear than the smaller.

Gear Inch Graph