Gain Ratio
Both "gear inches" and "metres of development" are concerned with the distance travelled per turn of the pedals, and are ultimately ways of indicating the mechanical advantage of the drivetrain, but neither of them take into account the length of the crankarm, which can vary from bike to bike. The crankarm is a lever arm. If two bicycles have different crank lengths but are otherwise identical, a longer lever arm gives a greater mechanical advantage. To take this into account, Sheldon Brown proposed a gear measurement system called "gain ratio," which is calculated by the distance travelled by the bike divided by the distance travelled by the pedals during one turn of the crank. He argued that it also has the advantage of being dimensionless.
Read more about this topic: Gear Inches
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