225/45/18, 245/45/18 i think mines are. The tyres are fine that size, Its more important about the quality of tyre used. zmanalex runs 600whp through this tyre size i think. If your cars set up for road then to be fair your limiting your track abilities. Even a set of lowering springs will make a difference and give you a more aggressive camber set up and center of gravity. I run 35psi on track. A softer tyre will cause the sidewalls to flex more, so maybe try making the fronts a couple of psi higher than the rear
Pm zmanalex or abbey motorsport as both of these guys know more than anyone else in britain about racing zeds probably
What type of tyre have you on just now for the track?