305/25 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 - 25 is height of the tyre, which is calculated as 25% of the tyre width (305mm), which means the sidewall height has 76.25 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 215/60 R17
(2167.07 mm), 285/30 R21
(2212.94 mm), 215/80 R15
(2277.65 mm), 285/40 R22
(2471.81 mm), 225/45 R18
(2072.51 mm), 295/30 R18
(1992.4 mm), 225/55 R19
(2293.68 mm).
