How do you measure gear ratios? Gear Inches

Gear inch is a system for measuring gear ratios in bicycles.  The gear inch is the equivalent diameter of the drive wheel on a high wheel bicycle (also known as a penny farthing).  The same system is applied to modern chain driven bicycles by multiplying the diameter of the driven wheel with the chain gear ratio.  This gives us a convenient number which allows us to compare the gearing on different bicycles.  Typically the lowest gear on a mountain bike is 22-26 inches and the highest gear on a road bike is 108-110 inches.