Riken Ultra High Performance is a tyre made in total of 55 sizes, ranging from R17 to R17 - this particular dimension is R18, which means it fits to a 18 inch/457.2mm diameter wheel. 225/45 R18 is 20mm wider than the narrowest dimension (205) and -30mm narrower than the widest one (195). This tyre profile of 45 means the sidewall height of the tyre is 45% of the tyre width (225mm), which results in 101.25mm sidewall height. In 225/45 R18, there are 2 possible speed indexes for Riken Ultra High Performance - "W","Y", which are good for speeds up to 270, 300 km/h.