Take your book and run it up to the top of your inner legs. How to Measure Your Inseam Length. Used on mountain bikes, these tires also go by the names: 27.5+, 650+, 6Fattie, or even just "Plus tires". Find a nice wall. ISO / ETRTO (mm) English size (inch) French size (mm) 35-584… Bicycle rims and tires came in many different types and sizes before efforts were made to standardize and improve wheel/tire compatibility. Stand with your back against the wall, as flat as possible and with no shoes or socks. The International Organization for Standardization (ISO) and the European Tyre and Rim Technical Organisation (ETRTO) define a modern, unambiguous system of sizing designations and measurement procedures for different types of tires and rims in international standard ISO 5775. One thing that makes bicycle wheel sizes notoriously confusing … Place your feet shoulder width apart. For example: 27.5″ / 650b Wheels (ISO 584 mm).