Gear-inches is based on the old penny-farthing bike (like the one in my avatar). It was a way to compare the gearing of the new-fangled "safety" bikes (with sprockets and chains) to the "ordinary" bicycles. It's the diameter of the big wheel on an "ordinary" that would take you as far in one turn of the crank. Multiply gear-inches by pi (3.1416) to get the distance you'll travel in one turn of the crank...close to it, anyway.