I happened across this. There is a lot of really good information here
regarding different tire sizes, types, and more importantly load-inflation
tables for many tire sizes. Some really good reads here. Some points to note
is that light truck and XL tires actually require higher inflation pressures
to carry some loads. This is mostly due the heavier construction creating
more heat. Also note how P-metric tires obtain their max capacity at 35 psi
and XL tires at 41 or 42 psi. Above that you mostly change handling and
rolling resistance.

Dennis

http://www.tiresafety.com/images/Tire%20Replacement%20Manual.pdf