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 195/65 R16(2073.14 mm), 265/45 R18(2185.61 mm), 205/40 R17(1871.76 mm), 265/65 R18(2518.61 mm), 205/55 R16(1985.17 mm), 275/35 R18(2041.09 mm), 205/75 R16(2242.78 mm).