Originally Posted by Pratt

Of course, 'gear inches', like Grinches, are largely imaginary. The alert, or even not very alert, reader will note that the diameter of the wheel is not how far you travel for a revolution, that would be the circumference. Circumference equals pi times the diameter, pi being 3.14 etc. So one really has a choice of ratio, gear inches, or development the distance travelled for of revolution of the pedals. A further complication comes from considering the real, functional radius of the rear wheel, depending on the wheel size, tire size, and rider size. Bicycle designers and builders seem to take pains to make sure their work is as arcane as possible. Where would the fun be if every thing was logical and standardized?