79pmooney

I've never liked ratios. Too hard to remember and keep straight. I find the ancient and granted archaic system of "gear inches" is the easiest to remember and visualize.

OP, "gear inches" is simply the wheel an early bike rider on the high-wheeler of the 1880s was riding. (Also your kid's tricycle.) Both travel one wheel circumference with one complete pedal revolution. The bigger the wheel the farther you went. So riders talked about the "wheel" they were riding. (And racing started right away because that is whet we male humans do.) Everybody "got " that bigger wheels were faster. (And with the need to straddle that big wheel in those high-wheelers, small people needn't apply for jobs as pro bike racers.)

Then chains came along. Suddenly anybody could ride any "wheel" they wanted. Since talking wheel diameter was accepted, people just kept doing exactly that, except now that "diameter" was the diameter of the equivalent high-wheel. Calculating it is simple measuring the outside diameter of the wheel you are riding and multiplying by the gear ratio used. (700c and 27" wheels with normal road tires come out close enough to 27".)

Chainring Teeth / Cassette Teeth X Wheel diameter (27" for most of us) = Gear Inches.

Say 39 teeth in front, 18 teeth in back: 39 /18 X 27 = 58.5" (or a higher-wheeler of just under 5 feet).

Gears over 100" are considered "high". Gears in the 60s, 70s and 80s "normal" for flat ground. Gears down to the 20s for climbing. Below that for fully loaded touring climbing.

(If you ever deviate from the common road wheels to say 29ers or folding bikes you will see where gear inches is much more useful than "ratios". You and your much smaller wife or kid can ride different wheeled bikes; both in 72" gears and pedal the same speed.)

Ben