235/50 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 - 50 is height of the tyre, which is calculated as 50% of the tyre width (235mm), which means the sidewall height has 117.5 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 285/65 R17(2520.5 mm), 225/55 R17(2134.08 mm), 295/35 R21(2324.46 mm), 225/65 R17(2275.46 mm), 315/35 R20(2288.65 mm), 235/40 R18(2026.96 mm), 235/50 R19(2254.41 mm).