In America and Britain bicycle gearing figures are directly related to
the old high wheeler bicycles of the 1880 time period.
If you see a gear inches listed as 50" this is how far a 50" diameter
high wheeler front wheel would move in one revolution of the pedals,
which were directly attached to the front wheel spindle. The actual
distance moved forward would be the listed gear inches X Pi. For a
50" gear then the actual forward movement would be 50 X Pi or 157".
For a 100" gear the figure would be 314".
This is IMO a antiquated system as it is not at all intuitive and
unless you are an experienced bicyclist with some familiarity with the
concept then you cannot relate the figures to actual riding experience.
A approximately 20" low is the common low for a 24 through 30 speed
derailleur mountain bike or a loaded touring bike set up for touring
in mountainous terrain.
A 110" or so high is in the range used by a road racer, downhill racer
or quite athletic light sports bike rider for use in good conditions.
For specialized races such as time trials, or for someone who loves to
pedal as fast as possible downhill, the top gear might be closer to
125" but the rider risks knee problems if used inappropriately.
In Europe outside England, and in most of the rest of the world,
bicycle gearing is referred to as "Development" and is listed in
meters. It is the amount of forward movement the bike achieves in one
turn of the crank for the selected gear.
An close approximation translation between the two systems is as
12.5 gear inches is 1 meter Development.
20 gear inches is 1.6 meters Development.
25 gear inches is 2 meters Development.
50 gear inches is 4 meters Development.
100 gear inches is 8 meters Development.
To me this is a more logical system. I see signs that it is gradually
being adopted here too as I have recently seen some American bicycle
makers starting to use it.