325/30 R21 doesn't belong to the most popular dimensions for 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 205/65 R16
(2113.98 mm), 275/40 R19
(2207.28 mm), 215/45 R16
(1884.64 mm), 275/50 R20
(2459.87 mm), 215/60 R16
(2087.27 mm), 285/30 R20
(2133.14 mm), 215/75 R16
(2289.91 mm).
