305/30 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 - 30 is height of the tyre, which is calculated as 30% of the tyre width (305mm), which means the sidewall height has 91.5 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 255/30 R19
(1996.8 mm), 180/60 R17
(2035.12 mm), 255/40 R20
(2236.81 mm), 185/60 R16
(1974.18 mm), 255/55 R18
(2317.55 mm), 190/55 R17
(2013.13 mm), 255/70 R16
(2398.29 mm).
