I would inflate tires to something between what door jamb of vehicle specifies and the max inflation for that tire. I eyeball how it sits on flat concrete, whether I can slip a piece of paper under the edges, and notice that the sand on the street is sticking to the middle four inches or of the tire and not the outside inch on either side. Then I lower the pressure until the tire tread sits flat on the pavement. As long as you're still above what the door jamb says, you should be good.
Some people have recommended chalking the surface of the tires to help see how even an impression is left on the pavement.
Some tire manufacturers will tell you what load their tire can carry when inflated to a certain psi. I'm certain they all have this data, but it's been very hard for me to find it. BF Goodrich gave me some data points for a tire a couple years ago, but when I called them recently they would only say, "Use what's specified on the door jamb." Probably only going to be more difficult with the Firestone/Ford fiasco.
An empty pickup is heavy in front and light in back. I get the front tires sitting flat and then inflate the rears the same, even if they do wear a little more in the middle cuz of the light weight. That's what rotating tires is for! I don't want to have to inflate them just to throw something in the back. On very heavy loads I will increase rear pressure, but without manufacturer data, I'm just guessing how high to go.
Personally I don't worry about smooth ride in my truck. It's a truck after all and the modern ones do very well for themselves already. You'll get better steering and handling and gas mileage by inflating your tires to spec or even higher. Don't under inflate just to get a soft ride.
Karl Nyhus, Minneapolis, MN
1997 Pacific Green Ford F-150 SuperCab XL,
short box, 4.6L V8, 4x4, 3.08 axles, 5-speed manual tran,
LT265/75R16 load range D BF Goodrich All-Terrain T/A
(max 65 psi, currently running 43-50 psi cold)
VDO tachometer, Better Built toolbox,
Line-X spray-in liner, rust-proofed/undercoated,
argent styled steel wheels with "chrome" wheel skins