Gear inches

When the high wheeler or penny-farthing was the "ordinary" bicycle form, the comparative diameter in inches of the driven wheel was an indication of relative speed and effort.

When "safeties" replaced "ordinaries", chains and sprockets allowed small wheels to be turned faster than the pedal cranks.

This ignores factors that contribute to the actual rolling radius of the tyre and rim together: the vertical deflection of the tire above the contact patch and the difference between the manufacturer's stated size and the actual tire radius when mounted and inflated.

It can be calculated by the formula: or equivalently: Thus gear inches and development differ by a factor of π times imperial-to-metric conversion.

Up until 2023, the UCI limited junior riders to a rollout distance of 7.93 metres (~99 gear inches).

[5] Coupled with a 700c wheel (622 mm diameter) and a 23 mm tire size (which approximately adds 2×23 mm to the wheel diameter), this gives an approximate diameter of 0.645 metres (depending on various factors such as air pressure, tire and rim), resulting in an estimated rollout distance of: π ⋅ 0.668 m ⋅ 52⁄14 ~ 7.53 m 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.

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 of revolution of the pedal axle during one turn of the crank.