275/65 R20 doesn't belong to the most popular dimensions for summer tyres - there are 1 tyre models available, which can be compared to 155/60 R15, 170/60 R17, 160/60 R14. Let's break down the sizing - 65 is height of the tyre, which is calculated as 65% of the tyre width (275mm), which means the sidewall height has 178.75 mm. The wheel diameter is R20, which means it fits to a 20 inch/508mm diameter wheel. The total circumference of the tyre is 2719.05 mm, which is very similar to 275/65 R18(2559.46 mm), 255/70 R18(2557.88 mm), 235/85 R16(2531.81 mm), 265/75 R16(2525.53 mm), 265/70 R17(2522.07 mm), 285/65 R17(2520.5 mm), 265/65 R18(2518.61 mm). There are multiple speed indexes available for summer tyres in 275/65 R20: Q(160km/h), S(180km/h). The same goes for weight indexes: 121, 126.