255/35 R21 doesn't belong to the most popular dimensions for allround tyres - there are 0 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 (255mm), which means the sidewall height has 89.25 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 2236.5 mm, which is very similar to 235/65 R16(2236.5 mm), 255/40 R20(2236.81 mm), 255/45 R19(2237.13 mm), 215/65 R17(2234.61 mm), 285/40 R19(2232.42 mm), 235/60 R17(2242.47 mm), 205/75 R16(2242.78 mm). There are multiple speed indexes available for allround tyres in 255/35 R21: W(270km/h). The same goes for weight indexes: 98.