275/45 R21 doesn't belong to the most popular dimensions for summer tyres - there are 14 tyre models available, which can be compared to 155/65 R14, 155/70 R13, 155/65 R13. Let's break down the sizing - 45 is height of the tyre, which is calculated as 45% of the tyre width (275mm), which means the sidewall height has 123.75 mm. The wheel diameter is R21, which means it fits to a 21 inch/533.4mm diameter wheel. The total circumference of the tyre is 2453.27 mm, which is very similar to 275/40 R22(2446.67 mm), 275/50 R20(2459.87 mm), 265/70 R16(2442.27 mm), 275/55 R19(2466.46 mm), 315/40 R21(2467.41 mm), 265/65 R17(2438.82 mm), 265/60 R18(2435.36 mm). There are multiple speed indexes available for summer tyres in 275/45 R21: H(210km/h), V(240km/h), W(270km/h), Y(300km/h). The same goes for weight indexes: 107, 110.