Bicycle tires come in a bewildering variety of sizes.
To make matters worse, in the early days of cycling, every country that manufactured bicycles developed its own system of marking the sizes. These different national sizing schemes created a situation in which the same size tire would be known by different numbers in different countries. The traditional sizing systems are based on a measurement of the outside diameter of a tire. Unfortunately, evolution of tires and rims has made these measurements lose contact with reality. This size tire is very close to 26 inches in actual diameter.