Michelin Pilot Sport 4S is a summer tyre made in total of 51 sizes, ranging from R17 to R17 - this particular dimension is R21, which means it fits to a 21 inch/533.4mm diameter wheel. 305/30 R21 is 80mm wider than the narrowest dimension (225) and 20mm narrower than the widest one (325). This tyre profile of 30 means the sidewall height of the tyre is 30% of the tyre width (305mm), which results in 91.5mm sidewall height.
Summer
