Tire Pressure Theory-Compelling or Outlandish?

[Artidemic13]

Hey everyone-

Wanted to get some thoughts from the more experienced off-roaders and overlanders regarding a very curious approach to tire pressure that I just heard a week ago while wheeling up in Hollister Hills, CA. I was with several guys with multiple decades of experience both driving off road and, more importantly, running training courses in off-road driving and recovery for law enforcement, forest fire crews, and the military. What they told me was really surprising.

They all run their personal off-roading vehicles at very high psi for on-road and highway driving, and then air down to what almost everyone (on this forum and elsewhere) would consider on-road pressures for driving off road. So, for example: one guy driving his Tacoma on 35-inch BFG KO2s runs them at around 50 or 55 psi on the highway, then goes down to about 35 for off-roading. If he's on sand, he'll go down to 15 or 20 psi, and if he's doing serious rock crawling, he might go down to 25 or so.

For my E-rated 35x12.5x17 Toyo Open Country AT/iii's with a max psi rating of 65, he actually suggested I be running these at 50 or 55 for the highway, then airing them down to 35 for off road.

Suffice to say this runs counter to everything I've ever heard or read about tire pressure. But these weren't just any old dudes, they were pros. When I asked about uneven wear on an over-inflated tire, he brushed it off, saying that the steel belting inside the tire keeps the tread squared and in even contact with the road regardless (until you air way down). He said at the higher pressure on the road, you'll not only get better gas mileage, but have better handling and cornering and actually prolong the life of the tire. And he said that off-road, he believed that most people air down way too much, especially those with heavy loads.

I sort of gave things a go last week and just kept my tires at 35 psi while wheeling all day with them up in Hollister (on moderate trails, admittedly) and to be honest, I never felt like I was wanting for traction. But I don't have a ton of off-roading experience so I'm not sure I'm in a position to appreciate the nuances. Certainly, I'm aware of the benefits of airing down when rock-crawling (allowing the tire to squish and wrap around rocks and obstacles). So this all left me a bit stumped.

What are your thoughts? I'd be keen to know if anyone else approaches their tire pressure this way or if this just sounds crazy.

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[Metalhead]

I would think the high pressure on the highway would make for a hard ride especially over bumps and potholes. In my limited wheeling experience I never felt the need to air down. I keep the pressure at whatever the door sticker says. Even running 35 inch tires.

 

[Bandit’s Lair]

For a Toyota, or any other IFS vehicle, that
works as they are better suited for soaking up bumps and lumps in those vehicles. For a solid axle vehicle the lower air pressure helps with ride quality. I’m not gonna judge them for running what they run but I’d never run mine that high. Hiway and on road is 35-38psi for my 37” Milestars. Off-road depending on what the trail is like I’ll run 18-@22. This is only because I have standard duty wheels without bead retention technology. On a beadlock or beadgrip I’d be down 10-15 all day on a trail without worries. And I’d have one heck of a more comfy ride than they would at a higher psi. To each their own.

 

[Cansberry]

Depends a lot on the axle type. As others have said, for IFS, no issues. With my work trucks (military), they're all IFS so they stay higher than what I'd run in a Jeep. My pops has a Bronco and he also stays higher both highway and on trail. For me, 35-40 is the sweet spot on road, and 17-23 off road without beadlocks. With beadlocks, 10-15.

 

[shooterrex]

When I picked up my Gladiator from the dealer the air pressures were 39-40. I took a trip to VA on the highway. I aired down to 35 for the trip home. 35 is a little better ride and I don't notice any difference in handling. Haven't really gotten off road yet. Just around the farm in our hayfields etc.

 

[Zachanadandy]

Just because they teach doesn't mean they don't do dumb shit. Ride will be shit, they won't get any longer tire life. The fuel economy will still suck (its poor aerodynamics not tire pressure). Butt they can get by with it on road and off road when traction isn't bad... until it is. They didn't discover anything aside from over inflated tires aren't a problem on road (until it rains or good forbid snows), and they aren't a problem off road on easy trails.

 

[Wheelin98TJ]

So much more benefit to go lower off road. Huge increase in traction.

Sand dunes at 15-20? 15 is the max for Silver Lake. The DNR won’t let you in with 16 if they spot it and check it.

Rock crawling at 25 would be awful. Spinning tires everywhere.

 

[bd100]

It all depends.

For 32" LT-E, on road, at highway speeds, in the summer, while carrying a heavy load, I had to raise the tire pressure from 40 to 50 PSI because the back tires were getting noticeably warmer than the fronts due to the cargo and trailer hitch weight. Big improvement at 50 PSI.

(Side note: You needs a more serious compressor to get the tires up to 50 PSI. Something which clamps to the battery, instead of through the 12V accessory port.)

 

[fourfa]

If it’s dry, there are no hard trails at Hollister (with the one exception of Tank Trap which requires a lot more than a built jeep). Run whatever pressure you want there, doesn’t really matter.

Some of this may come from old tires like the original BFG KOs which derived their E ratings from internal pressure, not from a particularly thick carcass. I had a set of those on a Tacoma and the MPG was dogshit at less than 50psi. Proper modern tires designed from the ground up for off roading are not like that. These old pros may not be interested in updating their beliefs though.

 

[VA6489]

Tire pressure is always a discussion point. Understand the tire is part of the suspension. It flexes with imperfections in the road to help provide a smooth ride. There is much is much written about the chalk test to find the right on road cold tire pressure.

Off Road is a different matter. if you are aa tread lightly type, lower Air pressures help preserve the trails and provide more traction. You can google tire pressure foot print and see the difference at each PSI setting the size of the foot print. Simple example on my BFG 39s I have twice the foot print at 10 PSI as I do at Road Pressure of 35.

Off road I use a simple rule of thumb. OEM wheel/tire run 12-15 psi, 20 on NFS fire road.
35-37 inch tire 10-12 wheeling on rocks and 15-20 on NFS roads
40s 8-10 wheeling technical and 15-20 on NFS roads.

MY BFG 39s i run at 35 PSI on road on a 6800# JTRD. off road anywhere between 8 and 12 PSI.


Running over inflated on road leads to bad tire wear and short tread life. Lots of folks teach..... some even know what they are talking about. Much like a forum group.......

Check out these guys

https://i4wdta.org/
Course are available and much like the SNavy Seal Teams many try and few complete.

 

[Janster]

Wow….. totally different than what we did back years and years ago (2000-2007).
On 35x12.50’s on 8” wide wheels - I’d air down to 15psi for offroading (rocky crawlin’ and rocky terrain).

I’m curious if those guys running 50psi + on the highway ever did a chalk test?
There’s a reason why ’recommended psi’ exists.
Sure, if you change your tire width and/or wheel width, that recommended number might change a bit to ensure you get even tire wear. Again, the chalk test!

For my JT on OEM 33’s - it’s recommended to run 37psi. I’ve never done a chalk test and don’t need to. I run them 2-3lbs higher at 39-40psi. If I were to ever do any offroading outside of a dirt road….I’d air down to 15psi.

 

[ChrisNLA]

I'm at about 35 PSI on the highway for comfort sake. Seems to be fine so far for tire wear.

I tend to air down to roughly 17 PSI when offroad. That seemed to serve me well over the years.

*Forrest Gump voice* Well - that's all I got to say about that.

 

[Jrgunn5150]

I mean...

If the steel belts keep the tread square, therefore avoiding uneven wear on just the crown by overinflating...

Then overinflating doesn't give better MPG, because the steel belts keep the whole tread in contact with the road anyway.

So that's the first logic fault... Didn't really look any further.

 

[GuzziMoto]

Wow. But I do have to agree with @Jrgunn5150 , right off the bat their logic seems to contradict itself.
Brings to mind the old adage, "Those that can, do. Those that can't, teach".
Right up there with "To err is human, to really screw things up takes an engineer".

Jokes aside, you can do some basic math. The OP's tires have a Load Range of 121R, which means each tire can support 3,197 lbs at max pressure (56 psi). That means each psi in that tire is roughly supporting 49 lbs. Running that tire at 50 psi should be about enough to support 2,450 lbs. That is per tire. So that would be enough to carry 9,800 lbs if evenly spread across all four tires. Increasing that pressure to 55 psi would increase the load capacity to 2,695 lbs per tire, or a total of 10,780 lbs.
I really don't think you need that much air pressure in your tires. And I am confident that running that much air pressure in your tires will result in uneven wear. But there is nothing stopping you from trying it. Nothing bad should happen, aside from increased and uneven tire wear as well as reduced traction from less tire being in contact with the pavement.

I do have to wonder if these guys are really that stupid. I wonder if they were just messing with you. That could be what was going on. Or not.

 

[Wolf Island Diver]

Those guys are morons. It sounds like they’re extrapolating high load high pressure commercial tires down to lite duty trucks because they’re “big” tires. Dealerships do this all the time too and they’re idiots. They look at the sidewall and load rating which means jack shit on a lite duty truck. Even my bike tires, I ignore the sidewall and tune the PSI for my weight, riding profile, casing design and terrain.

Putting 50 psi on 35s on a Tacoma, I’m surprised isn’t exceeding the sidewall rating. I would say it’s dangerous. The high load tires on some military and industrial vehicles are totally different and designed to carry much higher pressures. The average fire truck weighs between 20 and 30 tons. Extrapolating this to passenger vehicles is the height of dangerous stupidity. In the FD we were trained to avoid facing these tires head on when fighting brake fires because the already higher pressures + additional heat can cause the steel belts to explode out from the tire. They’re completely different tires. When has anyone ever seen passenger vehicle tire chunks all over the interstate?

If you chalk test your own tires, and everyone should, you can see the contact patch. You’re never going to see something drastic like a little tiny contact patch in the middle. What you’ll see is the edges shrink or grow with higher or lower pressure. On my Gladiator, even with all the crap living on the truck, I run around 27-29 psi. I’ve always had very even wear. On oversized tires you ignore the vehicle placard and on any vehicle you always ignore the sidewall rating. There’s no real benefit to adding more pressure. It’s not like more is somehow better. If you’re carrying a load or have weight on the tongue, you may need more pressure than normal, but we’re talking a few PSI. I run 29-30 when towing and loaded.

There’s no school of thought that I’m aware of that calls for more pressure off road. I don’t even know if this is a thing in off road racing. The Humvee has a central inflation system to reduce pressure off road, not increase it. For years I didn’t air down because I didn’t have a reliable way to air back up. With my PowerTank, I now always air down on trails. The difference is amazing and it’s not even traction which does improve substantially with low pressure. Washboard roads will literally destroy your vehicle. It’s why most aftermarket suspension manufacturers moved away from heim joints for control arms and back to bushings. Anything you can do to reduce the transmission of shock into the vehicle is a good thing. I think about how much shock is transmitted into my enduro Mtb riding over rocks and roots. I have to replace bearing yearly. I can feel the shock in my body. Now add 4500lbs to that. Off roading wears out everything. Washboards disappear at <10psi. It’s so much better and you don’t have these violent stutters moving through the whole body of the truck. I can’t imagine airing up to 35-50 PSI and driving down a rutted out fire road. The truck would shake apart. The rear hard top would be loose. The rack system would be loose. Everyone would be miserable. Don’t do it.

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