235/45 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 - 45 is height of the tyre, which is calculated as 45% of the tyre width (235mm), which means the sidewall height has 105.75 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 155/60 R15(1781.28 mm), 245/30 R20(2057.74 mm), 160/60 R17(1959.73 mm), 245/40 R20(2211.68 mm), 165/70 R13(1763.06 mm), 245/50 R20(2365.62 mm), 175/65 R13(1752.07 mm).