255/45 R21 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 (255mm), which means the sidewall height has 114.75 mm. The wheel diameter is R21, which means it fits to a 21 inch/533.4mm diameter wheel. The total circumference of the tyre is mm, which is very similar to 175/75 R16
(2101.41 mm), 255/40 R17
(1997.42 mm), 185/55 R16
(1916.06 mm), 255/45 R20
(2316.92 mm), 185/75 R16
(2148.54 mm), 255/65 R16
(2318.18 mm), 195/50 R16
(1889.35 mm).
