Kumho Ecsta PS71 is a summer tyre made in total of 125 sizes, ranging from R16 to R16 - this particular dimension is R18, which means it fits to a 18 inch/457.2mm diameter wheel. 255/55 R18 is 60mm wider than the narrowest dimension (195) and 30mm narrower than the widest one (285). This tyre profile of 55 means the sidewall height of the tyre is 55% of the tyre width (255mm), which results in 140.25mm sidewall height. In 255/55 R18, there is only one possible speed index for Kumho Ecsta PS71 - "Y", which is good for speeds up to 300 km/h.
Relevant tyre tests in same or similiar dimension (255/55 R18)