185/45 R15 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 - 45 is height of the tyre, which is calculated as 45% of the tyre width (185mm), which means the sidewall height has 83.25 mm. The wheel diameter is R15, which means it fits to a 15 inch/381mm diameter wheel. The total circumference of the tyre is mm, which is very similar to 225/55 R19(2293.68 mm), 295/40 R21(2417.14 mm), 225/70 R16(2266.34 mm), 325/25 R20(2106.44 mm), 235/45 R17(2020.99 mm), 235/55 R18(2248.44 mm), 235/65 R18(2396.09 mm).