245/30 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 - 30 is height of the tyre, which is calculated as 30% of the tyre width (245mm), which means the sidewall height has 73.5 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 180/60 R17(2035.12 mm), 255/40 R21(2316.61 mm), 185/60 R16(1974.18 mm), 255/55 R19(2397.35 mm), 190/55 R17(2013.13 mm), 255/70 R18(2557.88 mm), 195/55 R20(2269.8 mm).