Imperial All Season Driver is a allround tyre made in total of 134 sizes, ranging from R13 to R13 - this particular dimension is R18, which means it fits to a 18 inch/457.2mm diameter wheel. 225/55 R18 is 80mm wider than the narrowest dimension (145) and -10mm narrower than the widest one (215). This tyre profile of 55 means the sidewall height of the tyre is 55% of the tyre width (225mm), which results in 123.75mm sidewall height. In 225/55 R18, there is only one possible speed index for Imperial All Season Driver - "V", which is good for speeds up to 240 km/h.