Nankang SP-9 Cross Sport is a tyre made in total of 27 sizes, ranging from R16 to R16 - this particular dimension is R21, which means it fits to a 21 inch/533.4mm diameter wheel. 315/40 R21 is 100mm wider than the narrowest dimension (215) and 0mm narrower than the widest one (315). This tyre profile of 40 means the sidewall height of the tyre is 40% of the tyre width (315mm), which results in 126mm sidewall height. In 315/40 R21, there is only one possible speed index for Nankang SP-9 Cross Sport - "", which is good for speeds up to km/h.