Tire Pressure How-To: Does one size fit all?

A Helpful Guide to Lowering Your BAR

Sometime in the early 1980s, I was on one of my first four-wheel-drive trips with my new Toyota Hilux. Stopping at the trailhead, everyone started letting air out of their tires. Clueless as to why I asked one of the guys, “Whatcha doing?” With his two-word reply, “Airing down,” I asked the next dumb question… “Why?” His brief explanation, which included something about traction, began a multi-decade edification on the merits of reduced air pressure. What I’ve learned is that in the same way the Inuit use many words to describe snow, there are numerous variables when it comes to calculating the exact tire pressure you’ll need in a given situation. In this Backcountry Skills, we’re diving into the why, when, and how much when it comes to lowering the bar.

TIP: The appropriate tire pressure will vary depending on rim diameter, tire aspect ratio, vehicle weight, and type of terrain. 

Today, when someone asks me how much they should air down I usually respond, “I have no idea,” and initiate a discussion on the variables that might affect my answer. Things to consider are wheel diameter and tire aspect ratio, type of tire and load rating, weight of vehicle, do they have standard wheels or beadlocks, their driving style, and type of terrain. The latter (insert mud, snow, sand, granite, etc.) can be broken down into an infinite number of subcategories, each with an ideal pressure based on any combination of the former.

Rim Diameter and Sidewall Aspect Ratio – Diagrams

Wheel Diameter, Sidewall, and Load Rating

Let’s first look at wheel diameter and width, as well as tire aspect ratio. During the last decade there has been a shift toward big wheel diameters (18s, 20s, and larger) and shorter aspect ratios. Vehicle manufacturers are guilty on all counts, but we must cut them some slack. In order to accommodate increased brake caliper size (bigger rotors, shorter stopping distances, safer cars) and reduce rolling resistance and mass (complying with stringent EPA and CAFE requirements), large wheel diameters and low-profile tires are a necessary evil. Otherwise, big-diameter wheels have limited backcountry value in my opinion.

Theoretically, a 33-inch tire on a 16-inch wheel should give you 8.5 inches of sidewall between the rim and terra firma. Wrong! The bead seat and flange increase overall rim diameter by about 1.5 inches (17.5 inches actual), which leaves us with 7.75 inches of sidewall. Additionally, you can deduct another inch (+/-) for rolling diameter vs. manufacturer specs, which drops sidewall height to a still-respectable 7.25 inches. If we do the math for the same tire on a 20-inch wheel, we end up with a paltry 5.25-inch sidewall.

TIP: Large-diameter wheels equate to a shorter sidewall, and they limit the ability to reduce tire pressure. 

Load rating also affects how the tire reacts to reduced pressure. A tire with a load rating of E will have more plies, a higher maximum psi, and a stiffer sidewall than a C rated tire. Having said this, a lighter vehicle can get away with a softer, more flexible tire.

Deformation vs. Ground Clearance

You might ask why is sidewall height important? Reducing air pressure allows the tire to deform, or flatten slightly, which increases the contact patch. In loose sand, as well as soft snow and mud, this larger contact patch allows for greater floatation. In rocky terrain the softer tire will conform to obstacles, putting more rubber on the tractive surface. Think, sticking your pinky finger into a marshmallow versus a baseball. The marshmallow is soft and compliant while the baseball will break your finger.

TIP: Floatation is key when traversing soft sand and snow.

To demonstrate this effect, we’ve evaluated two tire/wheel combos. The first is a set of 35-inch BFG KM3 Mud Terrains (load range C), on 15×8-inch Marsh beadlocks (dedicated trail rig, approx. 5,000 lbs). The second is LT295/70R17 BFG KO2 All Terrains (load range E) wrapped around 17×8.5 Method 301 alloys (Ram 2500 diesel, approx. 6,000 lbs). Starting with street pressure, we reduced psi in increments while recording contact patch size and deformation.

You will note at highway pressure the contact patch for both is appropriately small, but when reduced to 20 psi, 15 psi, and 10 psi it becomes notably larger. This increase equates to more rubber on the ground, which equates to more floatation and/or deformation. One might argue that reducing tire pressure reduces ground clearance, which is true, and there are many safety concerns connected with reduced tire pressure. However, when choosing an appropriate psi for a given situation (review the aforementioned variables), lowering the bar is a game changer.

Example: While in Western Australia a few years ago I learned of a couple whose car became mired in the sand. The husband tried to walk out for help while the wife stayed with the car. Both perished from exposure, and when the recovery team arrived, they simply aired down the tires (increased footprint) and drove the vehicle out. Sad but true.

TIP: Reducing air pressure increases a tire’s footprint, its ability to deform in uneven terrain, and its ability to grip the tractive surface.

How Much?

During a recent fundraiser snow run someone blurted out on the radio, “Everyone should air down to between seven and nine pounds!” I shook my head in dismay and glanced at my long-time wheeling buddy Matt (also shaking his head), who was driving a new F-150 with 20-inch wheels and fairly low-profile tires. Our new ‘air referee’ had 40-inch tires on beadlock rims and the suggested pressure would be fine. But at the suggested pressure Matt’s dogs would be flat as a sailor frog, probably come off the rim, and possibly be damaged in the process. So, the million-dollar question is how much should we air down? I can guarantee that one size does not fit all.

I will be the first to state there are significant risks to running reduced pressure (legal disclosure here), which include losing a bead, damaging a tire and/or rim, having an accident, bodily harm, death, dismemberment, and mayhem of all forms. This is why tire and automobile manufacturers won’t suggest anything but full street pressure in their official literature. But theory and reality, as you know, seldom share common ground.

TIP: Reducing air pressure increases the risk of damaging the sidewall or rim, or losing a bead and causing an accident. Be cautious and take it slow.

We have seen that at 10 psi, my Ram 2500 nearly flattens a 33-inch tire on a 17-inch rim, but at 20 psi the footprint size almost doubles and there is enough sidewall for cautious driving (no hard turns or I risk losing a bead). I reserve 15 psi or less for I’m-in-a-pickle moments, then air back up immediately. During Baja treks (hard-pack dirt, sand, mud, rock, etc.) I drop my Tacoma’s 265/75R16 All Terrains to 15 psi and leave them there for the entire trip (short pavement sections at 55mph max) reserving 10 psi and lower for emergency situations. Lastly, my 35-inch KM3s on 15-inch beadlocks are perfectly happy at 7 to 8 psi on technical terrain like the Rubicon, Moab, or Fordyce.

TIP: One size does not fit all when it comes to air pressure.  

The reason is that even at this ultra-low pressure there is still plenty of sidewall for ample deformation and the beadlock mechanically clamps the outer bead of the tire to the rim—this eliminates losing a bead or spinning the rim within the tire under heavy torque scenarios. The caveat is that few beadlocks are DOT approved for on-road use.

Over the years I have experimented with different tires on different vehicles in different conditions, and I highly suggest you do the same. Try dropping to 60 percent of street pressure, take a look at the sidewall, turn the wheel and observe how the tire reacts, drive it a bit, and repeat the process until you find a pressure that works for the given terrain.

TIP: Reduce tire pressure in increments, evaluating sidewall deformation and tire performance at each level.

Air Up Air Down

I might be a bit old-school, but I typically air down by removing the valve core. This low-buck method will drop a 35-inch tire from 35 to 15 psi in less than a minute, but does require a core removal tool, accurate low-pressure gauge, and watchful monitoring…or you will be pulling out your compressor. Whether or not you use this technique, it’s a good idea to carry extra valve cores and stems in case you tear a stem or drop a core—I’ve done a contortionist act with a finger over the stem and the other hand gently mining the sand, mud, or snow for the core.

This leads us to the need to air up at the end of the trail. Most folks these days are using electric compressors, but before electrics were available the hot ticket was an engine-mounted belt-driven unit. They are highly efficient, last forever, and the higher you rev the mill the more air they produce. I run a York piston-style unit on my CJ-7, and a converted rotary-style AC compressor on my `82 Toyota Hilux. But for my other rigs, I have a portable ARB Twin electric that I move from vehicle to vehicle as needed.

TIP: Buy a high-quality compressor, an accurate low-pressure air gauge, and don’t forget extra valve cores and stems. 

While a single line and air chuck work great, there are dozens of deflation/inflation systems available. They consist of a manifold and five air lines; one for each tire and one that feeds from the air compressor. I’ve used MORRflate’s Quad+ unit with good results. Simply hook it to the compressor’s NPT fitting, the other leads to the tires, and monitor the built-in air gauge.

With a high-quality compressor, air-ups are easy-peasy and I usually air down as soon as I hit the dirt. Why, because the improved ride and reduced rattling of crew and cargo makes the day much more enjoyable. The increased footprint and improved grip will also help reduce tire spin, which will extend tire life and be easier on the trail and environment.

TIP: With most tires, reducing pressure within the 15- to 20-psi range will improve off-pavement ride quality. 

The Wrap-up

When I think about 20-something me asking, “Whatcha doing,” or the Air Referee’s “seven and nine pounds” comment I don’t pass judgment. Heading into the backcountry, whether for the first time or hundredth time, there is always a new skill to learn or improve upon. I’ll be heading to Moab next month, followed by a week in Sonora’s Gran Desierto de Altar and three weeks in Baja, nearly all on the dirt and aired down. Can’t wait to see what these next adventures teach me.

See you on the trail,
Chris

Editor’s Note: A version of this story previously appeared in the May/June 2023 print issue of Tread Magazine.