Kumho Ecsta PS71 is a summer tyre made in total of 125 sizes, ranging from R16 to R16 - this particular dimension is R21, which means it fits to a 21 inch/533.4mm diameter wheel. 255/40 R21 is 60mm wider than the narrowest dimension (195) and 30mm narrower than the widest one (285). 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 R21, 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 (255/40 R21)