Originally Posted by

**HillRider**
The most common way Americans express bicycle gear ratios is "gear-inches" which goes back to the old high-wheel bikes of the 1800's. A gear-inch value gives the same distance per crank rotation as a wheel of that diameter would. For example, a 54 gear-inch ratio gives the same travel distance per revolution as a wheel 54" in diameter.

Gear inches are calculated by multiplying the number of teeth of the chainring by the wheel diameter and dividing that value by the number of teeth in the rear cog. For simplicity both 27' and 700c wheels are assumed to be 27" in diameter and MTB wheels are 26". So if you are in a 42T chainring and a 17T cog on a 700c wheel roadbike your gear inches would be 27*42/17 = 66.7

Smaller gear-inch values are easier to pedal but you go slower at the same cadence. Larger gear-inch numbers are harder to pedal but you go faster. So you climb in small gears and use larger ones on the flats or downhills.

You can do the calculation for all of your chainring and cog combination and build up a table to see how they compare and where the overlap and duplicates are and to see the effect of changing to other size rings and cogs. The Sheldon Brown web site referred to above has a gear-inch calculator that makes this easy.