235/55 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 (235mm), which means the sidewall height has 129.25 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 R17(2020.99 mm), 235/55 R18(2248.44 mm), 235/70 R16(2310.33 mm), 155/80 R13(1816.47 mm), 245/40 R17(1972.29 mm), 165/65 R13(1711.23 mm), 245/45 R20(2288.65 mm).