Firestone Roadhawk is a summer tyre made in total of 154 sizes, ranging from R15 to R15 - this particular dimension is R18, which means it fits to a 18 inch/457.2mm diameter wheel. 255/55 R18 is 90mm wider than the narrowest dimension (165) and 40mm narrower than the widest one (295). 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 Firestone Roadhawk - "W", which is good for speeds up to 270 km/h. In 255/55 R18, Roadhawk usually competes against RainSport 5, which is a tyre from Uniroyal with a rating of 62%, compared to 57% of the Firestone in question.