325/30 R21 doesn't belong to the most popular dimensions for premium tyres - there are tyre models available, which can be compared to
325/25 R20,
155/60 R15,
155/65 R13. Let's break down the sizing - 30 is height of the tyre, which is calculated as 30% of the tyre width (325mm), which means the sidewall height has 97.5 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 mm, which is very similar to 295/40 R21
(2417.14 mm), 225/70 R16
(2266.34 mm), 325/25 R20
(2106.44 mm), 235/45 R17
(2020.99 mm), 235/55 R18
(2248.44 mm), 235/65 R18
(2396.09 mm), 155/80 R13
(1816.47 mm).
