If you are wearing this many medals on your chest, do not go cycling in a thunderstorm.
Unless you have aspirations to become a conductor.
Bicycle gears measured in inches date all the way back to the Ordinary or Penny Farthing bicycle. The large drive wheel was usually around 50 to 60 inches in diameter.
When the first chain-driven Safety bicycles were built the manufacturers advertised them as having, for example, a 60 inch gear. In other words, the gearing was the equivalent of a 60-inch diameter drive wheel.
The formula for calculating a gear is simple; it is the diameter of the rear wheel, divided by the number of teeth on the rear sprocket, times the number of teeth on the chainwheel.
A bicycle with a 27 inch wheel, an 18 tooth rear sprocket, and a 48 tooth chainwheel, would have a 27, divide by 18, times 48, equals a 72 inch gear. The equivalent of a 72 inch diameter drive wheel on an Ordinary.
You can make a handy custom gear table on your PC using Microsoft Excel. Print this easy to follow step-by-step instructions in PDF format.
Gear tables are useful especially when you have multiple gears in comparing different rear sprocket and chainwheel combinations.
If you are new to the sport of cycling, you will soon learn that gears in the 90 to a 100 inch and above are hard to push. You will use gears in the 70 to lower 80s range for most of your riding; 60 inch and below you will use on the hills.
A rule of thumb is one tooth difference on the rear sprocket is the same as between three and four teeth on the chainwheel. If you shift down from a 52 chainwheel to a 42, you need to shift up 2 or 3 teeth on your rear freewheel cluster to be the equivalent of one step (or one tooth) lower. A gear table is useful to see where these changes are.