The roush cars all came with 275-40-18's... on 18 x 10" rims ( 45 mm et). They also came with 275's on 20 x 10" rims. Square setup. Worked good, except when it came to high hp. The folks with the 2010 Roush 540 H cars typ dyno'ed 518-525 rwhp, right outa the box. No traction in 1st, 2nd, 3rd gears. Didn't get traction till 4th gear ( 1:1 ratio)...spinfest. That's starting from a greenlight, street use. Although they came with the roush UCA setup, and stiffer front / rear suspension, sway bars, monotube strut/shocks, it's still not enough. Forget what kind of rubber roush used.
I measured my 275-40-18 Nitto NT-555's and the tread is exactly 9.75" wide (18 x 10 rim). My 285-40-18 Nitto NT-555's tread measures 10.0" wide. My MPSS in 305-35-19 measure 10.75" of tread width. (19 x 10 rim). So far, the wider tread = more traction. One spring, for whatever silly reason, I had 36 psi (early evening) in the rear 275-40-18's.... no traction.
Dropped it down to 32 psi, all was well. I notice a big difference between cold pressures at 5:30 AM..and 4 PM. Also noticed a big difference if car was parked E-W,
and pass side is facing south. Pass side (sitting in the sun) pressures were higher than driver's side. I think 28-30 psi cold, in summer time may be optimum, since it will rise a bunch during the day.