225/50 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 (225mm), which means the sidewall height has 112.5 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 215/75 R16
(2289.91 mm), 285/40 R22
(2471.81 mm), 225/45 R17
(1992.71 mm), 295/30 R18
(1992.4 mm), 225/55 R19
(2293.68 mm), 295/40 R21
(2417.14 mm), 225/70 R16
(2266.34 mm).
