305/25 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 (305mm), which means the sidewall height has 76.25 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 mm, which is very similar to 235/35 R19(2032.92 mm), 235/45 R20(2260.38 mm), 235/60 R16(2162.67 mm), 155/60 R15(1781.28 mm), 235/85 R16(2531.81 mm), 160/60 R17(1959.73 mm), 245/40 R19(2131.88 mm).