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I’m not an expert however if you are asking my thoughts...

Your dealing with different plys in tires and most importantly different terrain. Sand vs trail vs rock. Bead lock?

I’ve seen some posts here guys going down to 10. To me that’s pushing it. I have done and feel comfortable with 15 in sand and 20 on trails. That’s just me.

 

Never seen any formulas for it and I dont think a formula would work since not all tires are made the same even within the same ply rating.

Your experiences may vary but maybe mine can help you out. I ran the same 265/70r17 KO2 C rated tires on 17x8" wheels for around 1.5yrs until recently replacing them about a month ago with 285's. I ran into different terrain conditions affecting how low I could drop the PSI. If it was muddy out then dropping to 18psi or lower would regularly cause mud to get into the bead causing a slow leak, staying around 20 psi the bead would typically hold and keep a good seal preventing mud from getting into the bead(even at 20 psi I still had issues with mud getting in the bead occasionally). The mud in teh bead wasnt bad but meant I would have to get it cleaned out. I would typically only lose 1-2 psi a day when this happened. One time I drove 800+ miles with this and lost around 4 psi over the 15hr drive. If I was in dry conditions I could get down to 16 without any issue. I never pushed lower than 16 and usually just ran around 18 so if/when the tires cooled it wouldn't typically drop below 16 psi.

 

The ideal gas law formula is PV=nRT
So it’s (P)ressure x (V)olume = (n)umber of (R)ocks x (T)ire size, or something like that.

Naw, its a “by the feel” type of thing. Some manufacturers post recommendations too. I only go down to 20-25 and no lower on my KO2’s. But mine are E’s, and stiffer sidewalls like mine need more air pressure support to prevent popping the bead than softer tires. Rebeading a tire on the trail is a huge hassle, even with the strap and pop methods (I haven’t tried the latter though, as I have a fear of burning the forest or myself).

 

Typically 1/2 your street pressure is about right.

Like others have said, what that number is depends on a bunch of factors, including what tires you’re running, the terrain, and the weight of your rig.

On my TJ, I ran my Swamper LTBs (34x10.50 on 8” wheels) at 4 psi off-road. Street pressure was 8 psi. Over more than 1k mikes off-road I only lost a bead once.

 

Hahaha! I work with that formula all the time and that's the first time I heard r = rocks!

OP, off road tire pressure is a function of experience, not science. If you don't have much experience, pick 18 psi. It's a good number to start with and learn from. You won't lose a bead and bending a rim is tough at that pressure. Have fun!

 

Well, what do you guys have against cute little moles?

 

I always air down to about 10psi. I lost a bead once but I think that could have happened even at 20 lbs. Turning into a rock while trying to drive up it popped the bead.

 

There is no formula, there are great benefits to running tire pressures low off road but with that comes the risk of spinning the tire on the wheel and popping the bead. If you plan on running your pressures lower then manufacturer recommendations then you should be adequately prepared. This may include bead locks or an air compressor. Possibly a cheetah tank. Some guys even use hair spray and a lighter to pop beads back on, but I wouldn't recommend that

 

I wouldn't recommend that either. Starter fluid works so much better!

 

Bravo. You will learn what 18 will give you versus stock, with regard to ground pressure. Then you can go to 15, or 22 and pretty soon you will know what you need based on experience. Have fun!

 

With sand you have to go even lower than you would for dirt, especially with E rated tires. On medium to hard packed dirt I run 20. In sand I go 10-12. 10 with my E rated tires & my light truck still doesn't seem like it's low enough but I haven't tried lower just to be safe. With Ds 12 worked fine for me. You want the tires to buldge pretty good which shouldn't be a problem with Cs but you have a little less meat on the tire with 70s, I'd try 15 to start on the sand, then go lower if needed.

 

Snag up a portable compressor off Amazon.

$98 Viair 88P Portable Compressor Kit (12V, 120 Psi, Forup to 33" Tires)

Viair 88P or 400P if you have larger tires.

 

Got a 12v compressor? One of the best is a Viar or ARB brand, but its pricey. I use a Smittybilt high volume single stroke one, and even HF has one for weekenders.

Edit: always just too late I am... roguegs is right, check out those Viars too. You will be popular on the trail with that as a filling station, too.

 

OP, given your tires, lower it to 15-18 PSI hot; you'll be both safe and will have the improved traction (& comfort) that you're after. There's no magic formula.

Oh, and get tire deflators (like the Staun ones) as they will make your life much easier.

 

I have a Viair 400P on the buy/sell here, but shipping to Hawaii would probably be too much.

 

Drive on your defkatde tires back to the gas station if you need to, especially if you're only going down to 18lbs. A co2 tank works well too.

 

I typically go to 10-12psi. Running HARD in rocks, I've burped air at 9psi but I've never lost a bead. Once the pressure falls below 18psi tire performance really increases off-road.

 

For added insurance for popin a rim, I carry an air pump, tire gauge, and ratchet strap that will span the circumference of my tires.

 

The YouTube channel (hear me out before ) 4xoverland has a couple of videos dedicated to tires and tire pressures. It's a South African guy names Andrew St. Pierre White who is an absolute legend in the global overland and 4x4 community. Seriously.

Anyways - one of his videos where he and another professional driver talk about tire PSI, they bring up a damn good point about heat. You can effectively lower your PSI until the tire tells you its too low. To check that, lower your tire pressure to whatever you want, lets say 12 psi. Go do your thing off-road for 15-20 min. Get out, and put your hand on the tire. If its hotter than a fresh cup of coffee, your pressures are too low for the terrain and your speed, and the weight of the vehicle. Either air up or slow down. If it's warm or even hot to the touch, but you don't have to pull your hand off because its uncomfortable, then you're doing good.

Also - in the same guy's "How to drive a 4x4" video (https://www.youtube.com/watch?v=TOweeVYkyps) there's a point where he talks about tire pressure vs what terrain you're driving over, and actually gives a ballpark percentage to use as a starting point. Something like ... soft sand/snow - 40%, which means 40% of your highway PSI.

Hope that helps.

(edited for spelling)