Here's my response from another related thread here:
Deflating for comfortable ride
I had the opportunity to speak with an individual (who shall remain nameless) within the BFG corporation. This particular gentleman was an engineer who helped develop a lot of the tires we run today. He told me (officially) that he can not condone running their tires at a lower PSI than recommended. However, (unofficially) he confirmed that running at a lower PSI
under the right circumstances would not degrade the integrity of the tire itself.
That being said; there are several factors to consider when selecting a proper PSI... this was some of his input on the subject:
1) Wheel size versus overall tire size.
So how much meat (rubber) do you have between the rim and the rock you're about to hit? If you're running 15" wheels with 35 or 37 inch tires you can go down in the 10 PSI range with no issues. If you're trying to run 18" wheels then 20 PSI is probably as low as you want to go if there's a chance you'll tag a rock. The reason for this is you can actually damage the interior of your tire if you were to pinch the tire between a rock and the rim. This type of damage will go unnoticed until you either remove the tire, or have a blowout...
2) Speed.
The rate at which the tire deforms and reforms can have an adverse affect on the sidewall as well. If you're running 55 MPH down a paved road to a trail head at 18 PSI you're generating a ton of heat due to the constant deforming and reforming of the tire footprint. It doesn't take long to do irreparable damage to a tire under these conditions (which is also hidden to the user). Consider airing up even if it's just a short run down a hard surface at speed.
3) Surface.
This factor goes hand-in-hand with speed. The type of surface you're running on can make all the difference on PSI selection. For example, if you're running on sand, the surface itself is helping support the integrity of the tire since it is also deforming to accept the footprint of the tire. You can typically run much lower PSI on sand due to this fact. Rocky trails at slow speeds are also good candidates for lower PSI since you want more deformation to avoid a puncture or cut. Gravel roads would be a bit less forgiving especially if they are hard packed. Consider a medium range PSI.
4) Aggressiveness of steer.
Basically this means; how hard will you been cutting the wheel at speed? You can run 2-5 PSI in sand... in a straight line. If you plan on doing donuts then you're probably going to blow a bead... and possibly roll your rig if you're carrying enough speed. Same for rock crawling, if you're at an extremely low PSI and try to cut hard while in a bind, you're likely to blow the bead.
The final decision on PSI is really up to you. It's your rig, so experiment with different settings until you find those sweet spots for your unit's wheels-to-tire size ratio and final weight. I highly recommend investing in some quality deflators (Stuan or ARB) and a 100% duty rated air compressor (Dual compressor ARB unit is incredible) to save a lot of headache and time. Also, get a quality air pressure gauge... that free one from NAPA ain't no good! (After tons of research I got a digital craftsman unit from amazon)
And if you do blow a bead... try a heavy duty ratchet strap (kept in the tool kit) for wrapping around the tire while re-seating.
Here are my typical PSI settings for our 2014 4Runner running BFG KO's in 275/70R17 Load Range E:
Every day travel: 40 PSI front 42 PSI rear (45 PSI in rear if loaded)
Highway between trails: 30-32 PSI (Used this in Colorado since our route had highways in between trail heads)
Gravel Roads: 20 PSI (24 PSI in rear if loaded)
Rocky Trails: 16 PSI (18 in rear if loaded)
Sand: 14 PSI (18 PSI in rear if loaded)
