215/45 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 (215mm), which means the sidewall height has 96.75 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 225/65 R17(2275.46 mm), 315/35 R20(2288.65 mm), 235/40 R18(2026.96 mm), 235/50 R19(2254.41 mm), 235/65 R16(2236.5 mm), 155/70 R13(1719.08 mm), 245/35 R19(2054.92 mm).