Here's how I see it:
I consider it fact that the MFG recommended pressures are only for the stock tires and are typically setup to cover the maximum recommended load. For my purpose on my stock tires on my unloaded truck that means they are too high and ride like a basketball for my day to day driving. Recommended is 29F/29R. I run 28F/25R.
The max pressure on the tire sidewall, any tire sidewall, has zero to do with the proper PSI for the vehicle. It is just that, the max before the tire could be in risk of rupture. It is also typically the PSI where the tire can carry the most weight. If you are running anywhere near this pressure on an unloaded Tacoma, you are driving a basket ball.....bounce -bounce - bounce. You also have very little rubber on the ground and likely have reduced traction and possibly safety.
A given tire is designed to flex a certain amount and operate best with a certain number of square inches on the road. You get the correct contact patch for a given vehicle by adjusting the PSI based on the weight bearing on that axle. To much air equals not enough flex and to little contact area, thus reducing traction and possibly safety. To little air creates to much flex. To much flex can greatly increase tire temperatures leading to tire failures. This is what happened in the Ford Explorer tire issues.
Math facts that pertain here:
If you take the weight on a given axle, divide it by 2, divide that by the PSI and you will know how many square inches of tire are touching a hard even road surface.
With all the above, if you are running larger than stock diameter or width tires, it's a pretty safe bet your correct PSI will be lower than the stock PSI.
I have heard rumor that all tire mfg publish charts for this purpose. However, I have never been able to locate one. If anyone has access to this info, I would love to have a copy. With out that the calk method here makes sense to me but should be used with due caution.
Here is what we can do ... you may or may not want to do this. Take the max load rating of the tire, divide by the max PSI, this equals the square inches that will be on the ground with these figures. Now take the weight bearing of your axle, divide by two (for one tire), divide by the square inches above, and you will have the correct PSI for that tire on that axle with your axles weight.
Ain't math wonderful.
Now does anyone know how much weight our various trucks have on each axle?