There are a number of factors involved, and depending on what you are trying to optimize, the ideal pressure varies. The "LT tires need more air pressure" article referenced above is worth reading, but it focuses on load carrying capacity to the exclusion of everything else, which is a one-dimensional approach to a multi-dimensional problem.
In general, you need to consider:
- load capacity. C and especially E rated tires need more pressure for the same loading than a P rated tire. Why? Because the stiffer sidewall generates more heat for the same amount of flex. Sidewall flex is almost entirely a function of pressure, (not stiffness), so you need more pressure to reduce flex to keep the heating the same.
That being said, the typical weight of a 4runner is far below the max load capacity of an E rated tire, so you don't need to inflate them to max pressure in most situations.
- Fuel economy. Again, stiffer sidewalls heat up more at the same pressure, so cost you mileage. Higher pressures will help you out there.
- Tire wear. Even tire wear across the tread requires optimizing the pressure for an even footprint. It's a pretty good bet that the optimum pressure for tire wear is less than that for best economy, particularly for an E-rated tire loaded lightly.
- Road grip and handling. This gets even more complicated, but too high pressures will reduce contact patch area. Pressures that are too low increase sidewall roll and squirming, negatively affecting handling.
- Ride quality. Lower pressure = softer ride.
Given all that, there is no "best" pressure. Make sure you have enough pressure to achieve adequate load capacity for your particular vehicle weight, at highway speed, and make sure you don't compromise handling to the point of being unsafe. Beyond that, experiment to optimize what you consider important.
I run my 255/75/17 C-rated BFG KO2's at 40 psi. At that pressure, ride quality suffers somewhat, but it's good on packed snow, and great for fuel economy. I'm not a performance driver by any means, so handling seems fine to me. So far tire wear across the tread seems even, but I'm keeping an eye on it. I would expect the centers to wear a bit faster than the outsides.
