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 245/40 R19(2131.88 mm), 165/70 R13(1763.06 mm), 245/50 R19(2285.82 mm), 175/65 R13(1752.07 mm), 255/30 R19(1996.8 mm), 180/60 R17(2035.12 mm), 255/40 R20(2236.81 mm).