Origin of The Term
When the high wheeler was the "ordinary" bicycle form, the comparative diameter in inches of the driven wheel was an indication of relative speed and effort. A 60 inch wheel propelled a bicycle faster than a 50 inch wheel when both were cranked at the same cadence. The technology of the high wheeler imposed a natural limit—a 60 inch wheel was about the maximum size that could be straddled by ordinary sized legs. When "safeties" replaced "ordinaries," chains and sprockets allowed small wheels to be turned faster than the pedal cranks. As result, a 28 inch wheel could be made to move a bicycle at the same speed as a 60 inch wheel. Such a bicycle was then said to be geared at 60 gear inches and pedalled similar to an ordinary with a 60 inch wheel. Thus on a bicycle geared at 72 gear inches one revolution of the pedals advances the bicycle the distance that a 72 inch wheel would in one revolution.
As a rule of thumb, a gearing in gear inches the same as a person's height in inches is a comfortable gear for riding on the flat. Conversion between gear inches and other equivalent measures such as metres of development is a simple arithmetic affair (though going metric in this case involves using that untidy transcendental number "pi"). If you know the gear inches, it is also a simple matter to calculate speed at any given cadence: gear inches multiplied by cadence divided by 336 equals miles per hour.
Read more about this topic: Gear Inches
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