255/60 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 - 60 is height of the tyre, which is calculated as 60% of the tyre width (255mm), which means the sidewall height has 153 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 275/45 R18(2213.88 mm), 215/50 R18(2111.78 mm), 275/65 R18(2559.46 mm), 215/65 R17(2234.61 mm), 285/35 R20(2222.68 mm), 225/35 R19(2010.93 mm), 285/45 R21(2481.54 mm).