255/45 R18 doesn't belong to the most popular dimensions for allround tyres - there are 2 tyre models available, which can be compared to 155/60 R15, 170/60 R17, 160/60 R14. Let's break down the sizing - 45 is height of the tyre, which is calculated as 45% of the tyre width (255mm), which means the sidewall height has 114.75 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 2157.33 mm, which is very similar to 255/40 R19(2157.02 mm), 255/35 R20(2156.7 mm), 215/65 R16(2154.82 mm), 225/45 R19(2152.31 mm), 235/60 R16(2162.67 mm), 295/30 R20(2151.99 mm), 295/35 R19(2164.87 mm). There are multiple speed indexes available for allround tyres in 255/45 R18: V(240km/h), W(270km/h), Y(300km/h). The same goes for weight indexes: 99, 103. You can find 255/45 R18 on 9 car models - mainly Audi, Bentley, Mercedes-Benz, Aston Martin, Cadillac.