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 255/60 R18
(2397.66 mm), 195/50 R15
(1809.56 mm), 265/30 R21
(2175.24 mm), 195/65 R14
(1913.54 mm), 265/40 R19
(2182.15 mm), 200/55 R17
(2047.69 mm), 265/55 R19
(2431.91 mm).