Some very good answers so far. I started writing this a while ago so there may be more since then!
This is one of those questions, don't you wish you could hear the answer to this very question from the perspective of someone like a Camel Trophy/Land Rover guy as well as from a Toyo tire manufacturer rep? I heard the answer to the topic question from a trainer from Land Rover at the Overland Expo in a class on tire pressure. I also then had the conversation with both the Land Rover guy AND the Toyo rep after the class was over.
The initial question was asked by the trainer to the crowd....."how low would you go if you were in conditions that warranted it, I.e. You are stuck in the Sahara in sand and need to get it going."
There was a lot of silence, and I finally spoke up and answered from the back of the crowd "it depends". He turned to me and said "depending on what?" I said, "it depends on the kind of tires I had on my vehicle and what vehicle I was in. I drive a 19,000 GVWR dually with 19.5" tires, which cannot be lowered essentially, at all." To which he said, "no. I would lower my air pressure to whatever it took to get out of the sand I was stuck in. Deal with getting it refilled if necessary later as soon as I got out of the situation."
I decided it wasn't worth interrupting him to try to explain about 19.5" tires (and the other .5" rims sizes/designs. So I nodded and let him go on with his class, which I have to say was EXCEPTIONALLY GOOD......I will fill in more about that class later. The point is I did not want to get into a side discussion about 19.5" tires and their inability to be lowered. Damn near nobody is overland exploration vehicles use .5" rim sizes.
He answered me and said "I would lower my air pressure to whatever it took to get out or to verify being completely unable to get out or get across the sand etc. The correct answer from the scenarios and tires he is knowledgable about.
The class was awesome, but during the class he pointed out the rep that was there from Toyo.....I then realize we are doing the class in front of the Toyo 18 wheeler marketing truck. I know this guy is going to know what I was talking about when I said 19.5" tires can't be aired down. I also know I almost bought 19.5" Toyos......can't remember the number but 608z or something? So I bide my time and enjoy the awesome class, and hang out afterwards, and eventually it's just me and the Toyo guy and the Camel Trophy guy. I answer to the instructor about how 19.5" tires have a different rim edge.....has a different angle, and I think a groove that holds the bead. I ask the Toyo guy to confirm that you literally can't lower the pressure in the 19.5" tire effectively at all it at least below about 70-80 PSI which doesn't help you extricate yourself from being buried in the sand. He says to both of us, "yep, he's right". The Camel Trophy guys says "huh. Oddball tire..." Exactly right. And not really applicable to Sahara desert exploring, but for me, CRITICALLY important.
My point is the answer, for us greater than 1 ton or 1 ton + crowd....is "It depends!"
19.5" answer, something like 60 PSI? My Michellin 19.5's was 80 I think. Not sure about my Hankook DH01's that I ended up with.
Here's the thing: if I had 16's or the standard up to "one ton" tires/rims, I would take the Camel Trophy dude's answer to me, in front of the whole class:
"However low I had to to get out of the 'stuck in the sand' and worry about airing back up AFTER I got out." In other words, now I that am not trying to move 5 feet, but rather, run down the road. For that, you better freakin have a way to air your tires back up. But if this were the situation, heck yeah, "ID AIR IT DOWN TO WHATEVER IT TOOK TO GET OUT including if the dually tires were touching. Who cares if they touch, if you are moving 5'! If it gets you moving and you drive 5 miles an hour no problem. But the moment you get back to hard pack or PAVEMENT then I either you keep going at 5 MPH or you get it aired back up. Highway speed with the duallies touching, no way!
If you want to air down a dually, one ton or greater it depends on:
What tires? 19.5's, basically none. If I can't get out of the sand, it's the winch, or help from another vehicle, it's jacking it up and laying boards or rock or carpet or that stupid green AstroTurf or riding the dirt bike out to get help???
With 16's or any wheel capable of being lowered without de-beading (a much bigger issue) "as low as it takes even if the duallies are touching! That might even help floatation, but once they touch you might lose effectiveness.....in other words I'd got AT LEAST until the duallies touched, maybe more. To get unstuck or back to the main road!
Hope this reads OK, that little cell phone screen is tough!
