Here is a simple way of obtaining gear ratio numbers for any combination of wheel size, chainring size, and rear sprocket size.

The gear ratio number dates back to the days of the penny farthing bikes and relates to the diameter of the "penny" wheel. In other words, a 76 inch gear will carry you the same distance with one revolution of the pedals as a penny farthing with a wheel diameter of 76 inches. So, the higher the number the greater the distance travelled, and the greater the resistance felt in the pedals. To calculate the "roll-out" distance travelled with one revolution of your pedals, it is necessary to multiply the gear ratio by pi (approximately 3.142).

By working out the gear ratios for each combination of front and rear sprocket size on your bike, you will get to know the relative "size" of each combination and be able to use this knowledge to good effect on the road.

Note that in many cases nominal wheelsizes (eg 27", 700c) are not the real dimensions, so the only way to be sure of your wheel diameter is to get your tape out and measure it, and don't forget to include the tyre.