325/30 R20 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 - 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 R20, which means it fits to a 20 inch/508mm diameter wheel. The total circumference of the tyre is mm, which is very similar to 160/70 R17
(2060.26 mm), 245/40 R21
(2291.48 mm), 165/80 R13
(1866.73 mm), 245/55 R17
(2203.2 mm), 175/65 R15
(1911.66 mm), 255/35 R18
(1997.11 mm), 185/50 R16
(1857.94 mm).
