I got this from another thread on the forum.
Draw a line across your tire tread with chalk. Then, drive a few feet until some of the chalk is worn away. The chalk line should be worn away evenly over the length if your pressure is correct. If there is more worn in the center of the tread, you are over inflated. More worn on the outside portions, you are under inflated.
I tried it on my 265/70 17 silent armours and my ideal pressure is about 38psi.
2008 CRD QDII, OME HD, Pintlers wheels with 265/70/17 Goodyear Silent Armour's, Jeepinbyal UCA's. 4XGuard Front guard, Side Guard, belly guard, Matrix HC. GDE ECO Tune w/ delete.