Originally Posted by
General Geoff
Gear inches is how many inches you travel forward for one complete pedal revolution.
Someone take this myth out back and kill it. "Gear inches" refers to the gear ratio x wheel diameter, traditionally calculated off a 27" wheel. It's a throwback that has been around long enough that it makes a useful universal reference for gearing (apparently from the era of penny-farthings, when all gear ratios were "1:1", but multiplying this by the wheel diameter made no more sense then than it does now). The number of inches you move forward for a single pedal revolution is the gear ratio x the wheel
circumference - or 3.14 x the "gear inches" value.