255/55 R20 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 - 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 R20, which means it fits to a 20 inch/508mm diameter wheel. The total circumference of the tyre is mm, which is very similar to 235/45 R20(2260.38 mm), 235/60 R16(2162.67 mm), 155/60 R15(1781.28 mm), 235/85 R16(2531.81 mm), 160/60 R17(1959.73 mm), 245/40 R19(2131.88 mm), 165/70 R13(1763.06 mm).