275/25 R21 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 - 25 is height of the tyre, which is calculated as 25% of the tyre width (275mm), which means the sidewall height has 68.75 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 mm, which is very similar to 180/60 R17(2035.12 mm), 255/40 R20(2236.81 mm), 185/60 R16(1974.18 mm), 255/55 R18(2317.55 mm), 190/55 R17(2013.13 mm), 255/70 R16(2398.29 mm), 195/55 R20(2269.8 mm).