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 195/80 R15(2177.12 mm), 265/55 R19(2431.91 mm), 205/50 R16(1920.77 mm), 275/30 R19(2034.5 mm), 205/65 R15(2034.18 mm), 275/40 R19(2207.28 mm), 215/40 R18(1976.69 mm). You can find 225/40 R18 on 51 car models - mainly Mercedes-Benz, BMW, Opel, Seat, Alfa Romeo.