225/40 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 - 40 is height of the tyre, which is calculated as 40% of the tyre width (225mm), which means the sidewall height has 90 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 235/40 R19(2106.75 mm), 235/55 R17(2168.64 mm), 235/65 R17(2316.3 mm), 155/70 R13(1719.08 mm), 245/35 R20(2134.71 mm), 165/60 R14(1739.19 mm), 245/45 R18(2129.06 mm). You can find 225/40 R18 on 51 car models - mainly Mercedes-Benz, BMW, Opel, Seat, Alfa Romeo.