215/50 R18 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 - 50 is height of the tyre, which is calculated as 50% of the tyre width (215mm), which means the sidewall height has 107.5 mm. The wheel diameter is R18, which means it fits to a 18 inch/457.2mm diameter wheel. The total circumference of the tyre is mm, which is very similar to 235/45 R20
(2260.38 mm), 235/60 R16
(2162.67 mm), 235/85 R16
(2531.81 mm), 160/60 R15
(1800.13 mm), 245/40 R19
(2131.88 mm), 165/65 R15
(1870.82 mm), 245/50 R19
(2285.82 mm).
