255/70 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 - 70 is height of the tyre, which is calculated as 70% of the tyre width (255mm), which means the sidewall height has 178.5 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 225/65 R17(2275.46 mm), 315/40 R21(2467.41 mm), 235/40 R18(2026.96 mm), 235/50 R19(2254.41 mm), 235/65 R16(2236.5 mm), 155/70 R13(1719.08 mm), 245/35 R19(2054.92 mm).