What is a "gear inch"?

This mysterious term is used by bicycle geeks to summarize their grear ratios. It turns out to equal the number of inches that your bicycle moves forward each time you crank your pedal by 2 radians (about 114.5 degrees). By fluke, this number is very close to the number of centimeters travelled when crank is turned by 45 degrees.

There are web sites which tell you how to calculate this by counting teeth and measuring your wheel and tire.

Here is Luis's patented straight-forward method to compute "gear inches", for us metric-minded non-Americans:

Measure in centimeters the distance your bike travels when turning the crank exactly half way around.
Divide this number by 4.