Old mechanic's trick to figure out best tire pressure for your particular vehicle load:
Take a big piece of sidewalk chalk and make a nice wide stripe over a section of tread from sidewall to sidewall. Drive vehicle a few feet forward on smooth pavement, try to make two complete revolutions of the tire. See where the chalk rubbed off. If it all (or almost all) rubbed off, you are at optimum pressure.
If outer edges are rubbed off, add more air, if the center is rubbed off, take out some air.
Repeat until you find a good pressure. Add some extra air to the rear if you frequently change the rear load by hauling stuff to prevent overloading the tire.
Later,
Joe