It's the gear ratio times the nominal diameter of the wheel. Your gear ratio is 1.5:1. Assuming it's a road bike, the nominal diameter is 27 inches. You would h

It's the gear ratio times the nominal diameter of the wheel. Your gear ratio is 1.5:1. Assuming it's a road bike, the nominal diameter is 27 inches. You would have a 40.5 inch bottom gear. This is a rough, easy-to-calculate way of comparing gearing on different bikes, ones that don't necessarily have the same diameter wheels. There are better methods around, but this is probably the most popular. I imagine it's because the better methods involve harder calculations. edit: grammar, punctuation.