View Here : Tire Size Meaning
In the old days, a tire was a tire and the sidewalls were about as tall as the tire was wide. The term “aspect ratio” was unknown. But today, a 90/90-18 means that the tire has an aspect ratio of 90%. That is, if the tire is (nominally) 90 mm wide, and the aspect ratio is 90%, then the tire is 90% of 90 mm, or 81 mm tall.
What do the size markings on bicycle tires mean? To simplify everything,Â the best sizing to use is the ETRTO number as it is a universal standard and is based off the technical measurement of the tires.