245/35 R21 doesn't belong to the most popular dimensions for winter tyres - there are 5 tyre models available, which can be compared to 155/60 R15, 170/60 R17, 160/60 R14. Let's break down the sizing - 35 is height of the tyre, which is calculated as 35% of the tyre width (245mm), which means the sidewall height has 85.75 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 2214.51 mm, which is very similar to 225/55 R18(2213.88 mm), 275/45 R18(2213.88 mm), 285/30 R21(2212.94 mm), 245/40 R20(2211.68 mm), 215/75 R15(2210.11 mm), 245/45 R19(2208.85 mm), 275/40 R19(2207.28 mm). There are multiple speed indexes available for winter tyres in 245/35 R21: T(190km/h), V(240km/h), W(270km/h). The same goes for weight indexes: 96, 110.