Michelin Pilot Sport 4 S is a summer tyre made in total of 181 sizes, ranging from R18 to R18 - this particular dimension is R18, which means it fits to a 18 inch/457.2mm diameter wheel. 255/40 R18 is 40mm wider than the narrowest dimension (215) and 60mm narrower than the widest one (315). This tyre profile of 40 means the sidewall height of the tyre is 40% of the tyre width (255mm), which results in 102mm sidewall height. In 255/40 R18, 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 255/40 R18, Pilot Sport 4 S usually competes against Pilot Sport 4, which is a tyre from Michelin with a rating of 75%, compared to 74% of the Michelin in question.