Steven McFarlane asks "What is a gear inch?"
For any given chainring/sprocket combination,
Gear Inches = chainring diameter / sprocket diameter x wheel diameter.
ie:  52/12x27 = 117 gear inches
     42/24x27 =  47 gear inches
     28/32x27 =  24 gear inches
The term "Gear Inch" was coined when cyclists switched from the old
high-wheelers to the "safety" bicycles that resembled those of today s
that they could compare the gearing of the new bikes with the old.
The new bikes were single-speed fixed gear machines, similar to today'
track bikes.  The old ones were single-speed with no gears.
One pedal revolution created one wheel revolution.
On a high-wheeler, a cyclist rode a wheel that matched the length of h
legs.  A tall man would ride a bigger wheel than a short man, would
travel farther with each pedal revolution, and would effectively be in
higher gear.  He would want to choose a chainring/sprocket combo that
would result in a number that matched the diameter of the wheel he was
used to riding.
Or something like that.
--- WILDMAIL!/WC v4.11 
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