Correct, the first number is tire width, the second number is the percentage of the width that makes up the sidewall. So a 205/45 has a 92.25mm sidewall, and a 215/45 has a 96.75mm sidewall. That's why when you increase the wheel size you have to decrease the sidewall ratio and increase the width, so that you find a happy medium. (It is also nice having wider tires) A helpful site for plus sizing.... http://www.1010tires.com/tiresizecalculator.asp?action=submitEggYolkBill said:The height is a percentage of the width. A wider tire will be slightly larger in diameter.