255/50 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 (255mm), which means the sidewall height has 127.5 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 195/70 R15(2054.6 mm), 265/45 R20(2345.2 mm), 205/45 R16(1856.37 mm), 265/70 R15(2362.48 mm), 205/55 R19(2224.56 mm), 275/35 R19(2120.89 mm), 215/35 R18(1909.15 mm).