Bicycle 'gear inches' is equivalent to the diameter of the drive wheel on a direct-drive bicycle with pedals fixed to the wheel, or when the number of teeth on the chainring and the sprocket are the same: a 1 to 1 drive. Gear inches is used mostly for comparing one drive train ratio to another and has little physical basis on a modern bicycle with derailleur gears.
Metres of development has a physical basis: it is distance travelled with one turn of a crank.
Gain ratio is distance travelled divided by the circular distance travelled by a pedal.
Typical crank lengths: 170mm on road bikes and 175mm on mountain bikes.
Online gear inch calculator created by CycleSeven (2009-2013)