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.