42 was the previous model S.. 40 is whats on the door tag for the Plaid w 21" 19's run slightly higher so 42 makes sense.. Personally I run 43.5 all around on my 21's and have hit the speed limiter without issue and plenty stability. I did run 36 psi for a while when I first got the car for more comfort, but the car got extremely unstable at 165, the rear end wanted to spin out and the tires were clearly deforming at that speed.. It was very dangerous.. Since that day I never run under 40psi.. 43.5 cold all around is my goto for high speed stability resulting in stiffer sidewalls and no tire deformation at speed from the heavy car load..