265/30 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 (265mm), which means the sidewall height has 79.5 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 235/75 R15(2304.36 mm), 160/60 R15(1800.13 mm), 245/40 R18(2052.09 mm), 165/65 R15(1870.82 mm), 245/50 R18(2206.03 mm), 175/60 R15(1856.68 mm), 245/75 R16(2431.28 mm).