It would be 9mm difference overall in the diameter. The first number is the width of the tire in mm. The second number is the height of the sidewall based on a percentage of the width. And obviously the last number would be your rim diameter. So you take 205 x .45 (because it's a % it's .XX). That gives you the height of your sidewall 92.25. Then 215 x .45 = 96.75. Leaving a difference of 4.5mm. Since the tire is round, we need to account for 2 sidewalls between the ground and the car for clearance. 4.5mm x 2 = 9mm diffrerence in total height.