I'm running 18in M3-wheels all year long. I started with 2,1 and 2,0 for the road then I had my KW-DDC and went up to the 19in recommended pressures of 2,3 and 2,2.
On a dry track with normal temperatures, I will decrease by 0,4 bars front and back, to allow for heating.
On a wet track, pressures should normally be increased above recommended stock pressures. But in all honesty, I just kept driving as it started to rain, as my track time was limited... Ho wait, that's maybe why ended up spinning ??
