DerbysDad wrote: "I assume the (non-rhetorical) answer is "the car co.?!"
Yes.
"So, why do you increase the pressure to 2 lbs above "the car co.?!" recommendation? In other words, how do
*your* driving habits differ from how "the car co.?!" thinks that "specific model car" will be driven, such that you feel the need for 2 lb increase? "
In a lot of cases car mfgs specify pressures tuned more for ride than for handling, etc In my personal experience I've noticed that exactly at those specs, more shoulder wear. So I pump a few extra PSI into them and hit that sweet spot.
Still others recommend the "chalk" test, where you make chalk lines across the treads, drive, and adjust the pressure until the chalk line wears most evenly. Check the pressures at that point, and write them down as ideal for your machine.