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.

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Rim Diameter and Sidewall Aspect Ratio – Diagrams

Rim diameter and tire aspect ratio significantly affect side wall height and the ability to effectively reduce air pressure. Note the 35×12.5R15 (center) and 295/70R17 (33.3-inch, bottom) tires provide more sidewall than the 37×12.5R22 (top). The same 37-inch tire on a 17-inch rim will provide an additional 2.5 inches of sidewall, greatly improving its function in soft sand, snow, or rock crawling.

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.

Crossing soft, muddy rice paddies in Cambodia necessitated dropping air pressure on the Land Rover Defender to about 15 psi.

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.

Load ranges, which are molded into the sidewall, indicate number of plies and maximum air pressure ratings.

Reducing air pressure allows a tire to squat and provide a larger contact patch.

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.

Deformation, the result of reducing air pressure, allows the tire to conform to uneven terrain rather than bouncing over it. These examples are of an E rated BFG KO2 at street pressure (65 psi) and 10 psi.

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.

Selecting the correct pressure is critical in some environments. During the Expeditions7 crossing of Antarctic, the author and his team set the 44-inch floatation tires on their Arctic trucks between 4 and 7 psi depending on the current density of the snow. When they got stuck, they dropped them down to 1 psi.

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.

Sand also varies greatly in density depending on grain size and moisture content. While crossing Australia’s Simpson Desert in the Austal winter, which was fairly damp, they were able to surmount most of its 1,100 sand dunes with the tires set at between 10 and 12 psi.

Most tire manufacturers frown on reducing air pressure, as they state heat buildup can damage the laminated layers within. This would be true on extended highway use, but author Chris Collard has run for weeks on end at 15 psi through Baja’s dirt two-tracks year after year without any negative effect on my tires.

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.

Experimenting with tire pressure during a 2,000-mile run through the Yukon to the Arctic Ocean, the author found running the Toyo Open Country all terrains on his Ram Rebel at 25 psi provided a good balance between traction, handling, and ride quality.

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.

A quality low-pressure gauge and valve core remover are standard kit in all my vehicles.

It’s a good idea to carry a tire repair kit that includes extra valve stems and cores.

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.

Compressor options range from electric units such as the ARB Twin

to engine-mounted, belt-driven units such as the piston-style York

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

No matter how prepared we think we are, you can still find yourself ankle-deep in mud…as author Chris Collard did during ARB’s 40th anniversary trek across Australia’s Outback. But if everything went smooth, what would you talk about around the fire?

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

FREQUENTLY ASKED QUESTIONS

What factors influence the vehicle manufacturer’s PSI recommendation?

Key Factors Influencing Vehicle Manufacturer’s PSI Recommendations

Understanding the manufacturer’s recommended PSI for your vehicle’s tires is crucial for optimal performance and safety. Here are the primary factors that play into these recommendations:

Ride Comfort and Handling
- Tire pressure significantly affects how your vehicle feels on the road. Proper inflation ensures smooth and controlled handling, enhancing driver comfort and vehicle stability.
Load Capacity
- Vehicles are designed to carry a specific weight. The PSI recommendation considers the maximum load your vehicle can safely carry while maintaining the tires’ integrity and performance.
Traction and Tire Wear
- Adequate tire pressure ensures optimal contact between the tire and the road surface. This balance is essential for effective traction, minimizing wear and tear, and extending tire life.
Fuel Efficiency
- Correct tire inflation can improve fuel economy. Under-inflated tires create more rolling resistance, which requires more energy (or fuel) to move the vehicle, affecting overall efficiency.

Essential Tip

Always refer to the specific PSI guidelines found in your vehicle’s owner manual or on the label inside the driver’s side door jamb. Check tire pressure before any long journey as readings can temporarily increase after extended driving. Proper maintenance not only enhances performance but also ensures safety on the road.

Why should you check tire pressure when the tires are cold?

Why Check Tire Pressure When Tires Are Cold?

When you’re checking tire pressure, it’s crucial to do so when the tires are cold. Here’s why:

Temperature Fluctuations: Tire pressure can change by about 1 PSI (pound per square inch) for every 10-degree Fahrenheit change in temperature. When tires are warm from driving, the heat can cause the air inside to expand, leading to higher pressure readings than those you’d get from cold tires.
Consistency and Accuracy: For the most accurate measurement, it’s best to check tire pressure when your tires are at ambient temperature – that is, they haven’t been driven on for at least three hours. This ensures that the pressure reading reflects the true state of inflation without the influence of the heat generated by driving.
Safety and Performance: Proper tire pressure, typically checked in cold conditions, is essential for maintaining safety and vehicle performance. Under-inflated or over-inflated tires can affect handling, fuel efficiency, and tire lifespan.
Manufacturer Recommendations: Most vehicle manufacturers base their recommended tire pressure guidelines on cold tire readings. By measuring when your tires are cold, you align with these guidelines, which can enhance vehicle stability and tire longevity.

Remember, keeping your tires at the proper pressure not only improves safety but also optimizes your driving experience.

Why is maintaining proper tire inflation important for performance and mileage?

Enhanced Tire Longevity

Keeping your tires properly inflated boosts their longevity significantly. When tires have the right air pressure, they wear evenly and slowly, maximizing their lifespan. This means you won’t have to replace them frequently, saving you money in the long run.

Optimized Performance

Proper tire inflation is crucial for maintaining optimal vehicle performance. Well-inflated tires ensure that your car handles smoothly and responds precisely to steering inputs. Whether you’re maneuvering through tight corners or cruising on the highway, correct inflation helps your car perform at its best.

Improved Fuel Efficiency

One of the lesser-known benefits of maintaining the right tire pressure is improved fuel efficiency. Under-inflated tires create more rolling resistance, which forces your engine to work harder and consumes more fuel. By keeping your tires properly inflated, you enhance fuel economy and reduce trips to the pump.

Safety on the Road

Safety is paramount, and correct tire inflation plays a pivotal role. Under-inflated or over-inflated tires can lead to uneven wear or even complete tire failure. This could potentially result in a dangerous blowout. By regularly checking and maintaining your tire pressure, you help ensure a safer driving experience.

Prevention of Damage

Improper tire pressure can lead to internal damage that isn’t immediately visible. This damage can eventually result in sudden tire failure. Routine checks and adjustments prevent these issues, keeping your tires—and you—safely on the road.

Maintaining the correct tire pressure is a simple yet vital task that enhances performance, extends tire life, saves fuel, and ensures your safety on every drive.

What is the difference between the maximum inflation pressure on the tire’s sidewall and the recommended tire pressure?

Understanding Tire Pressure: Maximum vs. Recommended

When it comes to tire pressure, there are two key numbers you need to understand: the maximum inflation pressure noted on the tire’s sidewall and the recommended tire pressure provided by your vehicle’s manufacturer.

Maximum Inflation Pressure

Location: You’ll find this number embossed on the tire’s sidewall.
Purpose: It indicates the highest PSI (pounds per square inch) the tire can safely handle.
Consideration: Running your tires at this maximum level can lead to premature wear and may negatively impact your vehicle’s handling and braking performance.

Recommended Tire Pressure

Location: This is typically found in your vehicle’s owner manual or on a sticker inside the driver’s door.
Purpose: Engineers determine this pressure to ensure optimal performance, safety, and longevity of the tires.
Benefits: Helps improve fuel efficiency, maintains strong traction, and contributes to a smooth ride.

The Key Difference

While the maximum pressure reflects the tire’s upper limit, the recommended pressure caters to the balance of ride comfort, vehicle handling, and fuel economy. Maintaining the correct recommended pressure is crucial for overall vehicle safety and performance.

How does temperature affect tire pressure?

How Temperature Influences Tire Pressure

Temperature plays a significant role in affecting tire pressure, and understanding this relationship is crucial for proper tire maintenance.

Cool Temperatures and Tire Pressure Drops

When the temperature drops, the air inside your tires becomes denser, leading to a decrease in pressure. Specifically, for every 10-degree Fahrenheit drop in temperature, tire pressure decreases by approximately 1 PSI (pound per square inch). This is why it’s essential to check tire pressure more frequently in colder weather to ensure safe driving conditions.

The Science of Heat and Expansion

Conversely, when temperatures rise, the air expands, increasing the tire pressure. This can lead to overinflation if not monitored carefully, potentially affecting the tire’s performance and longevity.

Best Practices for Accurate Tire Pressure

Morning Checks: Always check your tire pressure when the tires are cold, ideally in the morning before you start driving. This ensures that your readings are not influenced by the heat generated from driving.
Regular Monitoring: Make it a habit to regularly monitor your tire pressure, especially during seasonal changes when temperature fluctuations are more pronounced.

By staying vigilant and understanding how temperature impacts your tire pressure, you can maintain optimal tire performance and safety throughout the year.

Where can you find the vehicle manufacturer’s recommended PSI for your tires?

To find the manufacturer’s recommended PSI (pounds per square inch) for your vehicle’s tires, you have a couple of reliable sources you can check.

First, look at the sticker located on the driver’s side door frame. This sticker typically displays the tire pressure specifications along with other important information about your vehicle.

If you’re unable to find the sticker, your vehicle owner’s manual is the next best source. This manual provides detailed guidance on the recommended tire pressure for both front and rear tires, ensuring your vehicle’s optimal performance and safety.

Remember, maintaining the correct tire pressure is crucial for your safety, vehicle performance, and fuel efficiency.

How often should you check your tire pressure?

Keeping an eye on your tire pressure is crucial for vehicle safety and performance. It’s recommended to check your tire pressure at least once a month. This routine maintenance ensures that your tires remain in optimal condition, as they naturally lose air over time.

Why Monthly Checks Matter

Natural Air Loss: On average, tires lose about 1 PSI (pound per square inch) each month. Regular checks help you catch and address this gradual deflation.
Optimal Performance: Properly inflated tires support your vehicle at its maximum load and contribute to better fuel efficiency.
Safety First: Regularly checking your tire pressure can prevent unexpected issues on the road, such as blowouts or uneven tire wear.

Quick Tips for Tire Pressure Checks

Use a Reliable Gauge: Invest in a quality tire pressure gauge for accurate readings.
Consider Temperature Changes: Check more frequently in extreme weather conditions since temperatures can affect tire pressure.
Consult Your Vehicle Manual: Always refer to your vehicle manufacturer’s recommendations for the ideal PSI.

By integrating monthly tire pressure checks into your routine, you can enhance your driving experience and ensure your vehicle runs smoothly.

What is the best tool for checking tire pressure?

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

A tire pressure gauge is your go-to tool for ensuring your tires are properly inflated. Many drivers mistakenly believe the correct inflation pressure is marked on the tire’s sidewall. However, what’s listed there is the maximum inflation pressure, not the ideal level for everyday driving. Keeping your tires at this maximum PSI could lead to faster wear and compromised traction or braking.

A quality low-pressure gauge and valve core remover are standard kit in all my vehicles. Regular use of these tools will help maintain optimal tire health and performance. Make sure to invest in reliable tools to ensure accuracy in your readings and adjustments.

Why is it important to not inflate tires to the maximum PSI on the sidewall?

Inflating tires to the maximum PSI listed on the sidewall can lead to quicker wear and may negatively impact traction and braking performance.

Where can you find the correct inflation pressure for your tires?

The correct inflation pressure can typically be found in your vehicle’s owner manual or the sticker inside the driver’s door jamb, not on the tire itself.

What tool should be used to check tire pressure?

A tire pressure gauge is the essential tool for checking tire inflation levels accurately.