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 285/60 R18
(2510.76 mm), 225/55 R16
(2054.29 mm), 295/35 R20
(2244.67 mm), 225/65 R16
(2195.66 mm), 315/30 R22
(2349.28 mm), 235/40 R17
(1947.16 mm), 235/50 R18
(2174.61 mm).
