Tire size specifications are communicated using a set of codes printed on the tire wall. These codes include the specifics of the given tire including diameter, aspect ratio and use type and can include other information such as tire construction, maximum pressure and speed ratings, among other properties. Wolfram|Alpha can be used as a tire size calculator and help show you how to read tire sizes. Many different types of tires are supported including car and truck tires, bicycle tires, and motorcycle tires.
Car and light truck tire specifications often start with a P or LT followed by standard codes to specify the properties of that tire.
Motorcycle tires follow rules similar to car and light truck tires and are often preceded by an MT or MC prefix.
Bicycle tires usually have a simpler specification that includes general descriptions of the tire dimensions.