Michelin Pilot Super Sport is a summer tyre made in total of 156 sizes, ranging from R17 to R17 - this particular dimension is R21, which means it fits to a 21 inch/533.4mm diameter wheel. 325/25 R21 is 120mm wider than the narrowest dimension (205) and -10mm narrower than the widest one (315). This tyre profile of 25 means the sidewall height of the tyre is 25% of the tyre width (325mm), which results in 81.25mm sidewall height. In 325/25 R21, there is only one possible speed index for Michelin Pilot Super Sport - "Y", which is good for speeds up to 300 km/h.