I'm unsure also, but it's all about rolling diameter and width if i am not mistaken. I calculated the differences using this tool (I recommend anyone else who changes wheels/tyres does the same):
http://www.rimsntires.com/specs.jsp
VS. OEM:
Fronts
4mm closer to suspension/brakes
6mm father sticking out in fenders
6mm additional wheelwell fill
1.8% faster on speedometer
Rears
3mm closer to suspension/brakes
7mm father sticking out in fenders
4mm additional wheelwell fill
1.3% faster on speedometer
An 18" wheel package saw almost identical variances as the tyre width was upped from 215 to 225 and 245 to 255 respectively. The 19's only saw a step down in tyre wall height from 40 to 35 and 35 to 30 respectively.