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 195/75 R16(2195.66 mm), 265/50 R20(2428.45 mm), 205/50 R15(1840.97 mm), 265/75 R16(2525.53 mm), 205/60 R16(2049.58 mm), 275/40 R18(2127.49 mm), 215/40 R17(1896.89 mm). You can find 225/40 R18 on 51 car models - mainly Mercedes-Benz, BMW, Opel, Seat, Alfa Romeo.