Michelin Primacy 4+ is a summer tyre made in total of 122 sizes, ranging from R16 to R16 - this particular dimension is R18, which means it fits to a 18 inch/457.2mm diameter wheel. 255/50 R18 is 70mm wider than the narrowest dimension (185) and -10mm narrower than the widest one (245). This tyre profile of 50 means the sidewall height of the tyre is 50% of the tyre width (255mm), which results in 127.5mm sidewall height. In 255/50 R18, there is only one possible speed index for Michelin Primacy 4+ - "Y", which is good for speeds up to 300 km/h.