255/40 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 (255mm), which means the sidewall height has 102 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 275/30 R21
(2194.09 mm), 205/70 R15
(2098.58 mm), 275/40 R21
(2366.88 mm), 215/45 R17
(1964.44 mm), 275/55 R19
(2466.46 mm), 215/60 R17
(2167.07 mm), 285/35 R18
(2063.08 mm).
