Michelin Pilot Sport 4 S is a summer tyre made in total of 178 sizes, ranging from R18 to R18 - this particular dimension is R21, which means it fits to a 21 inch/533.4mm diameter wheel. 325/30 R21 is 110mm wider than the narrowest dimension (215) and -10mm narrower than the widest one (315). This tyre profile of 30 means the sidewall height of the tyre is 30% of the tyre width (325mm), which results in 97.5mm sidewall height. In 325/30 R21, there is only one possible speed index for Michelin Pilot Sport 4 S - "Y", which is good for speeds up to 300 km/h. In 325/30 R21, Pilot Sport 4 S usually competes against P ZERO, which is a tyre from Pirelli with a rating of 88%, compared to 99% of the Michelin in question.