Gear Inches / Gain Ratio
(chain ring/sprocket x wheel diameter) / (wheel radius/crank length x chain ring/sprocket)
Using default inputs
26 inch diameter wheel • 175 mm crank arm length (radius ratio = 1.89)
Gain Ratio = for every inch of pedal travel, the bicycle will travel Gain Ratio inches.
(Gain ratio concept and formula courtesy of Sheldon Brown - http://www.sheldonbrown.com/gain.html.)