265/40 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 - 40 is height of the tyre, which is calculated as 40% of the tyre width (265mm), which means the sidewall height has 106 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 155/70 R13
(1719.08 mm), 245/35 R19
(2054.92 mm), 165/60 R14
(1739.19 mm), 245/45 R17
(2049.26 mm), 170/60 R17
(1997.42 mm), 245/60 R18
(2359.96 mm), 175/70 R13
(1807.04 mm).
