When I hear these discussions I always think back to a fun little high school physics experiment:
figure out the car's weight by measuring air pressure in each tire and the square inches of contact patch of each tire.
Hope I don't bore anyone but I think its useful
The force each tire exerts on the ground is the psi in the tire times the area of contact. There are subtle variables but that's the important part.
If you air down offroad, the portion of the vehicle weight on each wheel is constant. The psi goes down. So the contact patch area goes up. Example:
1200 lbs supported on single wheel. 30psi.
1200 = 30 x a -> a = 40 sq in
Air down to 15 psi:
1200 = 15 x a -> a = 80 sq in
Thus why airing down keeps you afloat on soft sand! More contact area spreading out same weight.
So why high pressures wander on highway with 35"+ tires? Contact patch gets smaller. How? By making the center of the tread bulge out and lifting the side blocks off the ground. Its like standing still on one Rollerblade instead of a flat shoe. Big tires already spread the weight over a larger area than stock. So the only way the physics can hold true if you compare a 31" tire with 35psi in it to a 40x13.5" tire with 35psi in it is for the big tire to disproportionately load (aka bulge) the center of the tread more than the outer edges. That bulge makes that nice wide tire you have act like the Rollerblade...
Hope that made sense.
The most accurate way to find your ideal on road pressure for a given tire is unfortunately the most inefficient. You check tread depth on the rear axle. You drive it say 2500 miles. You precisely recheck tread depth at multiple spots across the tread. If you have less tread depth remaining at the center than the outer blocks, then your pressure is too high. If you are wearing the outer edges faster than the center, then tire pressure is too low. Assuming good alignment. I've actually done this informally to figure out ideal front and rear pressures on big tires on baja- inspired truck builds I've done in the past. The weight distribution on a diesel truck, for example, is so different that to make the tread wear evenly requires huge psi differences front to rear.... and it all comes back to...
contact patch area x psi = weight on that wheel
Here's a pic of one of my last projects, no lift (just huge fiberglass fender wells!) on Toyo MT 37s. 46 psi front tires, 27 psi rear. Wore beautifully. Just had to remember to swap pressures when I rotated tires!
