285/45 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 (285mm), which means the sidewall height has 128.25 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 255/70 R16(2398.29 mm), 195/55 R20(2269.8 mm), 265/35 R20(2178.69 mm), 195/70 R15(2054.6 mm), 265/45 R20(2345.2 mm), 205/45 R16(1856.37 mm), 265/70 R15(2362.48 mm).