285/50 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 - 50 is height of the tyre, which is calculated as 50% of the tyre width (285mm), which means the sidewall height has 142.5 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 185/65 R15(1952.5 mm), 255/55 R20(2477.15 mm), 195/45 R15(1748.3 mm), 265/30 R19(2015.65 mm), 195/60 R15(1932.08 mm), 265/35 R22(2338.29 mm), 195/75 R16(2195.66 mm).