Michelin Primacy 4 is a summer tyre made in total of 240 sizes, ranging from R15 to R15 - this particular dimension is R18, which means it fits to a 18 inch/457.2mm diameter wheel. 255/40 R18 is 90mm wider than the narrowest dimension (165) and 0mm narrower than the widest one (255). 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 Primacy 4 - "Y", which is good for speeds up to 300 km/h. In 255/40 R18, Primacy 4 usually competes against PremiumContact 6, which is a tyre from Continental with a rating of 83%, compared to 82% of the Michelin in question.