Firestone Roadhawk is a summer tyre made in total of 147 sizes, ranging from R0 to R0 - this particular dimension is R20, which means it fits to a 20 inch/508mm diameter wheel. 255/45 R20 is 50mm wider than the narrowest dimension (205) and 40mm narrower than the widest one (295). This tyre profile of 45 means the sidewall height of the tyre is 45% of the tyre width (255mm), which results in 114.75mm sidewall height. In 255/45 R20, there is only one possible speed index for Firestone Roadhawk - "W", which is good for speeds up to 270 km/h.