Originally Posted by FBinNY

This question starts with a false assumption, namely that "harder" tires (more width x more pressure) roll easier. They do if the road is glass smooth, but rolling resistance is only a single factor among many.

Traction is probably the next bigger factor, and increased pressure reduces traction significantly. Comfort is another, and so is the tire's weight and wind resistance. The goal is to find the best balance of all of these. Research shows that rolling resistance drops rapidly as tires move from low to high pressure, but as the pressure rises, the rate of improvement diminishes, while the other factors begin to worsen measurably.

Since this is a matter of balancing a bunch of variables, there's no single best number for everybody. Riders, especially light riders, on smooth roads will favor high pressures and narrower lighter tires. Heavier riders, or those on lousier roads, do better with more width (raising the rim higher off the road) and lower pressures.

You might look at the auto industry. While there's variation in a band, thire pressures tend to be held at 24-32psi, and the tire width increased in proportion to the weight of the vehicle. You could drive an SUV on the little wheels made for a Miata, if you jacked the pressure high enough, but the handling and traction would go to hell.

BTW- while the principles and math are easy, optimizing and designing accordingly are harder. Use any pressure coming from a chart as a starting place, and experiment on either side, looking for either the lowest pressure before you feel an uptick in rolling resistance, or the highest pressure before you feel skittish handling or "road fatigue" from bumps.

If you've selected the right width tire, both methods will yield a similar pressure.