Sorry but anything other than 215's look wank on Esc Cos rims on an RST imo.
I've had Esc Cos rims on 5 out of 6 of my S1's & I've had 215's on al of them without any probs at all.
i like the sound of proper width tyres lol, were your arches cut? if so i dont have an arch kit to get away with this
