I have a focus ST170 and i will be puttin my Focus RS 18 wheels on very soon. My car will be lowered with Eibach Pro springs kit very shortly to

At the moment the tyres are 225/40/18 but 3 need replacing as the will keep rubbing etc or so i have been told, which therefore leads to my question. Do i need a different size tyre?

I have been told that 215/35/18s are the right size ... yet when i looked in Performance Ford ST170 Modifying section is says 225/35/18.

I also have a sierra cosworth 4x4 which is completly standard but would i be able to use 215/35/18s on that to?

any help would be appreciated

andy