255/45 R18 doesn't belong to the most popular dimensions for tyres - there are tyre models available, which can be compared to 325/30 R21, 155/60 R15, 155/65 R13. 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 mm, which is very similar to 185/55 R14(1756.46 mm), 255/45 R19(2237.13 mm), 185/70 R14(1930.82 mm), 255/60 R18(2397.66 mm), 195/45 R16(1828.09 mm), 265/30 R21(2175.24 mm), 195/60 R16(2011.88 mm). You can find 255/45 R18 on 9 car models - mainly Audi, Bentley, Mercedes-Benz, Aston Martin, Cadillac.