285/30 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 (285mm), which means the sidewall height has 85.5 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 225/40 R19
(2081.62 mm), 295/25 R20
(2059.31 mm), 225/55 R17
(2134.08 mm), 295/40 R20
(2337.34 mm), 225/65 R17
(2275.46 mm), 315/40 R21
(2467.41 mm), 235/40 R18
(2026.96 mm).