Nick
Had my S1 for 9 years during which I did about 25,000 miles in it. Always had 225/40/18s on front and rear and never felt the need for anything different. Best tyres were the Vredsteins it ran on. Phenomenal grip, great wear and at the time they were a good deal.
Personally i would find something else to change rather than tyre tyre since I think the car is extremely well balanced as it is.
Cheers, Philip
