255/40 R20 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 (255mm), which means the sidewall height has 102 mm. The wheel diameter is R20, which means it fits to a 20 inch/508mm diameter wheel. The total circumference of the tyre is mm, which is very similar to 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), 275/50 R20(2459.87 mm). You can find 255/40 R20 on 12 car models - mainly Audi, ALPINA, Aston Martin, Mercedes-Benz, Nissan.