It depends on the offset of the wheel. With 18s, you will have to get a wheel with atleast a +45 or +48 offset. Anything lower and you will rub your fenders, guaranteed. Check out this thread:
They only look fine because the Ecsta MX (I had them in 215/40-17 and they looked great) has no rim edge protector.. my potenzas have a big rim guard so it makes it look like the tires are stretched onto the wheels.