255/30 R21 doesn't belong to the most popular dimensions for winter tyres - there are 0 tyre models available, which can be compared to
155/60 R15,
170/60 R17,
160/60 R14. Let's break down the sizing - 30 is height of the tyre, which is calculated as 30% of the tyre width (255mm), which means the sidewall height has 76.5 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 2156.39 mm, which is very similar to 255/35 R20
(2156.7 mm), 255/40 R19
(2157.02 mm), 255/45 R18
(2157.33 mm), 215/65 R16
(2154.82 mm), 225/45 R19
(2152.31 mm), 295/30 R20
(2151.99 mm), 235/60 R16
(2162.67 mm).
