255/55 R18 doesn't belong to the most popular dimensions for allround tyres - there are 9 tyre models available, which can be compared to 165/80 R13, 155/70 R19, 155/80 R13. Let's break down the sizing - 55 is height of the tyre, which is calculated as 55% of the tyre width (255mm), which means the sidewall height has 140.25 mm. The wheel diameter is R18, which means it fits to a 18 inch/457.2mm diameter wheel. The total circumference of the tyre is 2317.55 mm, which is very similar to 255/50 R19(2317.24 mm), 255/60 R17(2317.87 mm), 255/65 R16(2318.18 mm), 255/45 R20(2316.92 mm), 255/40 R21(2316.61 mm), 255/70 R15(2318.5 mm), 235/65 R17(2316.3 mm). There are multiple speed indexes available for allround tyres in 255/55 R18: H(210km/h), R(170km/h), T(190km/h), V(240km/h), W(270km/h), Y(300km/h). The same goes for weight indexes: 105, 109, 120. You can find 255/55 R18 on 10 car models - mainly Mercedes-Benz, Land Rover, Audi, BMW, Porsche.