@neilr_ I have the 18" wheels and Pirelli P Zeroes also, and also I'm further south than you in Kentucky. During the summer months, I start my tires out at 29 psi as directed by the manual. Depending on how hard I use them, they get up to 31 or so. On the race track, they start out at 26 psi in the cool morning air but make 29 or 30 on my way to the track for summer HPDE driving, and on the track they get up to about 32 psi. A senior instructor examined my tires after I managed to do a nice 360 in the grass and suggested, by not seeing appropriate sidewall wear, that I lower my pressures a psi which I didn't do because pictures of my car and tires on the track show the full width of tread being used. I probably should listen to someone with much more experience than I, but sometimes the teenager creeps out of this old guy. In winter, my condo's underground garage maintains about 60 degrees. I raise the tire pressures 1 psi/15 degrees difference of colder outside air, so I add about 2 psi to each tire for 31 psi and outdoor temperatures of 30 degrees. It seems to work out well. For what it's worth.