see the tech section, right column, bottom article I think is on tire pressures
my 38.5x14.5 SX's are rated at 3370# PER TIRE at 30 psi ....my rig weighs 4100# total.
3370# x 4 = 13480#
4100# / 13480# = .304
.304 x 30 psi = ~9 psi
I run mine at 10-12 psi road and 4-5 psi trail. Basically, you play with the pressures to get them to track well, some tires like lower, some higher. I used the above to get a starting point, then played with it from there.