It's actually 10 mm. The first numbers in a tire size are the section width of the tire in millimeters. The second number is the aspect ratio--the percentage of sidewall height in relation to the section width (e.g. a 50 series tire sidewall is 50% as tall as the tire is wide.) And of course the last number is the rim diameter in inches.

Not only will the 215 be slightly wider, but the sidewall will be slightly taller as well. Unless, of course, you go down a bit in aspect ratio--a 45 series vs. a 50 series.