Here's a scenario that plays out every weekend at trailheads across the country: two trucks pull up, same weight class, same size tires, same target PSI. One airs down to 22 and rolls through rocks like it's on rails. The other airs down to 22 and the driver spends the whole day white-knuckling through every obstacle, vaguely aware that something feels wrong but not sure what.
The difference isn't the vehicle. It's not the driver. It's not even the tread pattern.
It's two letters on the sidewall that most people have never bothered to read.
SL (Standard Load) and LT (Light Truck) tires are built for fundamentally different jobs. And when you start airing down -- when you're asking your sidewalls to flex and support thousands of pounds at reduced pressure -- that difference stops being academic and starts being the thing between you and a bad day on the trail.
Before we dive in, let's clear up something that trips up even experienced off-roaders. When you look at a tire, there are actually three different things going on at once:
These three things are independent. A tire can be Euro-metric XL (like the Rivian OEM Pirellis), P-metric SL (your basic sedan tire), or LT E-rated (a heavy-duty truck tire). The article you're about to read is mostly about #2 and #3 -- load range and construction -- because that's what determines how a tire behaves when you air down. But #1 matters too, especially if you drive something heavy. We'll get to that.
Before we get into why it matters, let's make sure you can actually find the information. Every tire has its life story printed right on the sidewall. You just have to know where to look.
Find the big number string on your tire. It looks something like this:
P265/70R17 116H XL
or
LT285/70R17 121/118S E
or
275/65R20 116H XL
Here's what you're looking for:
The first letter(s) tell you the tire type:
The numbers (265/70R17) are your section width, aspect ratio, and wheel diameter. Important for fitment, but not what we're focused on here.
The load index and speed rating come after the size. "116H" means the tire supports a certain load at a certain speed. Higher load index = more capacity per tire.
The load range letter (for LT tires) tells you the ply rating:
XL (Extra Load) means the tire has a higher load capacity than a standard SL tire of the same size. You'll find XL on both P-metric and Euro-metric tires. It allows a higher max inflation pressure (41-42 PSI vs 35-36 for SL) and therefore a higher max load. Don't confuse XL with LT. They're not the same thing -- XL is still passenger-type construction, just with more capacity.
The designation is molded into the sidewall, usually near the bead (closest to the wheel). It's not always easy to spot on a dirty tire. Crouch down, run your finger along the sidewall text, and find the full size string. If it starts with "P" or has no prefix, you're on a passenger-type tire (P-metric or Euro-metric). If it starts with "LT," you're on a Light Truck tire.
Quick check: If you don't feel like getting on the ground, pull up your tire's exact model online. The product page will list the designation clearly. But knowing how to read the sidewall means you can check any tire, any time -- yours, your buddy's, or the ones at the shop.
This is where it stops being letters and starts being engineering.
The sidewall is the part of the tire between the tread and the bead -- the vertical wall that flexes when you air down. How many layers of material are in that wall determines how stiff, how strong, and how much abuse it can handle.
SL/P-Metric tires: Typically 2 plies of polyester cord in the sidewall (rated as 4-ply equivalent in the old rating system). These sidewalls are designed to flex for a comfortable ride on pavement. They're lighter, quieter, and smoother at highway speeds.
LT tires: 2 to 3 plies of polyester or nylon in the sidewall (rated 6 to 10-ply equivalent depending on load range). These sidewalls are thicker, heavier, and more reinforced. They're designed to carry heavy loads, resist punctures, and maintain shape under stress.
This is where the numbers diverge sharply:
| Tire Type | Max Load-Rated PSI | Typical Max Load Per Tire |
|---|---|---|
| SL (Standard Load) | 35-36 PSI* | 2,200-2,800 lbs |
| XL (Extra Load) | 41-42 PSI* | 2,600-3,200 lbs |
| LT -- C-rated | 50 PSI | 2,200-2,800 lbs |
| LT -- D-rated | 65 PSI | 2,800-3,400 lbs |
| LT -- E-rated | 80 PSI* | 3,200-3,800 lbs |
* Note: Some E-rated tires on 16" wheels are rated at 65 PSI. Some SL tires have sidewall max pressures above 50 PSI.
Look at that max PSI column. An SL tire reaches max load at 35-36 PSI. An E-rated LT tire reaches max load at 80. That's not a small gap. That's a canyon.
And here's the kicker: the max load rating on the sidewall is only valid at the max load-rated PSI. Drop the pressure and the load capacity drops along with it. This is where the math gets real.
Before we get into the specific scenarios, you need to understand how tire load capacity actually changes with pressure. This is the part most people -- and most content online, honestly -- get wrong.
You'll see this everywhere: take the ratio of your target PSI to the tire's max PSI, multiply by the max load. Simple division. Intuitive. And wrong.
That linear method says: drop to 50% of max pressure, lose 50% of your load capacity. Drop to 70%, keep 70%. Sounds logical. But that's not how tires work.
The Tire & Rim Association (TRA) -- the same folks who write the standards for the tire industry -- uses a power curve formula:
Load at target PSI = Max Load x (Target PSI / Max PSI) ^ 0.585
That exponent (0.585) is the key. It means tires retain more load capacity at reduced pressures than a simple ratio suggests. The relationship is non-linear -- the tire's structure itself supports load, not just the air pressure.
Here's what that looks like in practice:
| % of Max PSI | Linear Method Says You Keep | TRA Formula Says You Keep |
|---|---|---|
| 70% | 70% of max load | ~82% of max load |
| 50% | 50% of max load | ~67% of max load |
| 35% | 35% of max load | ~55% of max load |
That's a big difference. At half your max pressure, you've still got two-thirds of your load capacity -- not half. This matters every time you make a decision about how far to air down.
Why it matters: The linear method consistently overstates the danger of airing down. Sounds like that should be the safe way to be wrong, right? But it creates two real problems. First, people think they can't air down as much as they safely can. A Rivian owner running the linear math might conclude that 25 PSI is dangerously low, stay at 35 on the trail, get less traction, and have a worse experience because the tire isn't conforming to terrain. Second, when experienced off-roaders spot wrong math, they dismiss the entire source -- including the parts that genuinely matter.
We'll use the TRA formula for all the calculations in this article. If you want to double-check the math yourself, there are TRA load-inflation calculators online, or you can plug the formula into any calculator with an exponent button.
On pavement at full pressure, the difference between SL and LT is mostly about ride quality and noise. SL rides smoother. LT is stiffer. Most people on the highway would prefer SL.
Off-road at low pressure, the difference is about structural integrity under load. And that changes everything.
SL sidewalls are thinner and more flexible by design. That flexibility is great for absorbing bumps on the freeway. But when you drop the pressure and ask those thin sidewalls to support heavy loads over rocks and roots, they flex a lot. Sometimes too much.
Excessive sidewall flex means:
This is the part that should make you pull out a calculator.
Let's use a real example. An SL tire rated for 2,800 lbs at 35 PSI (its max). Using the TRA formula, at 25 PSI:
2,800 x (25 / 35) ^ 0.585 = 2,800 x 0.833 = ~2,332 lbs per tire
Now picture a vehicle that weighs 7,000 lbs (loaded). Divided by four tires, that's 1,750 lbs per tire -- before you account for uneven weight distribution, cargo, passengers, or the dynamic loads of hitting bumps.
At 25 PSI on those SL tires, you have about 582 lbs of margin per tire on average. That sounds more comfortable than people think. But add a couple hundred pounds of gear in the back and account for uneven weight distribution (the front axle on most trucks and SUVs carries more weight), and the heaviest loaded tire might be supporting 2,000+ lbs. Now your margin on that corner is closer to 300 lbs -- and that's before dynamic forces from hitting bumps or dropping off a ledge.
It's not zero margin. But it's not a lot, either.
LT sidewalls have more plies and thicker rubber. They don't flex as easily -- which means they retain more structural shape at low PSI. A C-rated LT tire at 25 PSI still has noticeable sidewall stiffness compared to an SL at the same pressure.
More importantly, LT tires have higher max load ratings. Even at reduced pressure, you're starting from a bigger number, so you've got more headroom before you're in the danger zone.
Let's run the same scenarios with the TRA formula.
A C-rated LT tire rated at 2,800 lbs at 50 PSI, aired down to 25 PSI:
2,800 x (25 / 50) ^ 0.585 = 2,800 x 0.667 = ~1,868 lbs per tire
Compare that to the SL at 25 PSI giving us 2,332 lbs. The C-rated LT is still lower -- and that's intentional. C-rated LTs are designed for the same class of vehicles as SL tires. The real advantage shows up with D and E-rated LTs on heavier vehicles.
An E-rated tire at 3,500 lbs max at 80 PSI, aired down to 25 PSI:
3,500 x (25 / 80) ^ 0.585 = 3,500 x 0.547 = ~1,915 lbs per tire
For that same 7,000 lb vehicle, that's still a healthy margin above the 1,750 lbs average per tire. But the bigger story here isn't just the math -- the structural sidewall of that E-rated tire is physically stronger than the numbers suggest. The ply construction supports loads beyond what even the TRA formula implies, especially at lower pressures. And you're also not going to run an E-rated at 25 PSI on a 7,000 lb truck -- you'd run it higher (we'll cover the formula in a minute).
The real-world takeaway: LT tires maintain more sidewall structure at low PSI because of physical construction, not just air pressure math. The extra plies do work even when the air pressure isn't.
Every stock Rivian -- R1T and R1S -- comes from the factory on Euro-metric tires. Not P-metric. Not LT. Euro-metric.
This is true for the stock Pirelli Scorpions (Gen 1 and Gen 2) and the stock Goodyear Wrangler Territory ATs (Gen 2 only, starting 2025). Check the sidewall: you'll see no letter prefix. "275/65R20" -- not "P275/65R20" and not "LT275/65R20." That makes them Euro-metric, governed by ETRTO standards.
And here's the important part: the OEM Pirellis are rated XL (Extra Load), not SL. That XL designation means they reach their maximum load capacity at 41-42 PSI, not the 35-36 PSI of a standard SL tire. The load index on all current Rivian OEM tires is 116, which means a max load of 2,756 lbs per tire.
The door jamb says 48 PSI for on-road use. That's above the XL max load-rated pressure of ~42 PSI. This isn't Rivian cutting corners or doing anything sketchy -- it's standard automotive engineering. Many OEMs specify pressures above the tire's max load-rated pressure for handling, contact patch optimization, and range. On an EV this heavy, that extra pressure also helps with efficiency. The sidewall max on the OEM Pirellis is 51 PSI, so 48 PSI is well within the tire's safe inflation range.
But here's what that tells you: at 48 PSI, the tires are already at their full rated load capacity. There's no extra load capacity to unlock by going higher. The additional pressure above 42 PSI is for ride and handling characteristics, not for carrying more weight.
Now consider the numbers. A Rivian R1T weighs roughly 7,100-7,200 lbs. The R1S is in the same neighborhood. That's before you load the gear vault, put a rooftop tent on, or fill the frunk with recovery gear.
The GVWR (Gross Vehicle Weight Rating) is 8,532 lbs for both the R1T and R1S. That puts them in EPA Class 2b -- the same weight class as an F-250 or Silverado 2500. Let that sink in for a second.
Here's where it gets tight. Federal safety standards (FMVSS 110) require that when passenger-type tires -- both P-metric and Euro-metric -- are installed on trucks, SUVs, and multi-purpose vehicles, you have to divide the tire's load capacity by 1.1 to get the effective capacity. This is called the de-rating rule, and it applies to every Rivian on the road.
The math on the rear axle (which carries the most weight):
Twenty-five pounds. On the rear axle, at the rated pressure. That's the margin between meeting the federal safety standard and not.
For reference, the front axle is less critical -- GAWR of 4,134 lbs means 2,067 lbs per tire, well within the de-rated capacity.
Historical note: In early 2022, some Rivian 22" wheels shipped with Load Index 115 Pirellis (2,679 lbs max). After the 1.1 de-rating, that gives 2,435 lbs -- which is 45 lbs short of the rear axle requirement. Rivian corrected this to LI 116 across all sizes.
Using the TRA formula at 25 PSI on the XL OEM tires:
2,756 x (25 / 42) ^ 0.585 = 2,756 x 0.757 = ~2,086 lbs per tire
After the 1.1 de-rating: 2,086 / 1.1 = 1,896 lbs
Rear axle load per tire: 2,480 lbs.
That's a deficit of 584 lbs per tire on the rear axle at trail pressure. Even without the de-rating, you're at 2,086 lbs of capacity against a 2,480 lb load. The math doesn't work.
Now, in practice, the de-rating rule is a regulatory standard for minimum tire ratings, not a physical limit. Your tire doesn't suddenly fail at 2,505 lbs. And at 25 PSI, the actual load on the rear axle depends on what you're carrying and the terrain. But the numbers show why Rivian recommends 31 PSI as the off-road minimum for the 20" tires -- and why many owners who do serious trail work swap to LT.
This is where it gets real for Rivian and other EV owners. The OEM Pirelli Scorpion AT+ weighs roughly 39-48 lbs depending on the wheel size. Common LT E-rated alternatives in 275/65R20 weigh 54-65 lbs. That's potentially 60-104 lbs of additional rotating, unsprung mass across four tires.
On an EV, that extra weight hits your range. Estimates vary, but figure 3-7% range reduction depending on driving conditions. For a Rivian with ~300 miles of range, that's 9-21 miles. Not nothing -- especially on a road trip.
This is the real tradeoff: LT tires give you massively more load capacity and sidewall strength, but they cost you range and ride quality. That's a decision every Rivian owner has to make based on how they actually use the truck. If you're doing occasional easy dirt roads, the OEM tires at sensible pressures are probably fine. If you're doing regular trail work with gear, the LT upgrade is hard to argue against.
Any heavy vehicle on passenger-type tires -- loaded half-ton trucks, full-size SUVs with gear, overland-built rigs -- faces the same math. Rivians just happen to be the most visible example because they're heavy from the factory and they all ship on Euro-metric tires.
For comparison:
| Vehicle | GVWR | OEM Tire Type | Load Index | Notes |
|---|---|---|---|---|
| Rivian R1T/R1S | 8,532 lbs | Euro-metric XL | 116 | 25 lbs margin on rear after de-rating |
| Ford F-150 Lightning | ~8,250 lbs | Euro-metric | 115-116 | Similar tight margins depending on trim |
| Tesla Cybertruck | ~8,669 lbs | LT-metric, Load Range D | 123/120 | No de-rating -- LT tires are exempt |
| GMC Hummer EV | ~10,400 lbs | LT-metric, Load Range E | 126 | No de-rating -- LT tires are exempt |
Notice the pattern? The heavier EVs (Hummer, Cybertruck) ship on LT tires from the factory. No de-rating issue. The ones on Euro-metric tires (Rivian, Lightning) are right at the edge.
The tire industry is evolving to handle EV weights. ETRTO introduced a new designation in 2021 called HL (High Load) -- a step above XL, designed specifically for heavy vehicles like EVs. HL tires can carry more weight at the same inflation pressure as an XL tire of the same size. Some Rivian 22" tires may carry this marking. If the aftermarket catches up, HL tires could offer a middle ground between the tight margins of XL and the weight penalty of going full LT. Worth watching.
Not everyone needs LT tires. Here's how to think about it:
SL tires are fine if:
Consider LT tires if:
LT tires are strongly recommended if:
The key framing: check YOUR tire, not your vehicle. Two identical trucks at the trailhead can have completely different tire constructions. The question isn't "what vehicle do I drive?" -- it's "what tire is on my wheel right now?"
Not all LT tires are created equal. The load range letter determines how stiff the sidewall is and how much pressure (and weight) it can handle.
The lightest LT construction. Max PSI of 50. These are the closest LT tires to SL in terms of ride quality -- relatively flexible sidewalls, lighter weight, less road noise.
Best for: Vehicles under 6,000 lbs. Jeeps, Tacomas, Broncos, 4Runners, mid-size trucks. C-rated is the off-road sweet spot for these rigs -- you get LT construction and puncture resistance without the stiff, heavy ride of higher load ranges.
Airing down: C-rated LTs flex nicely at trail pressures. You don't need to drop as far to get good tire conformity.
The middle ground. Max PSI of 65. Stiffer sidewalls, more load capacity, heavier.
Best for: Heavier half-ton trucks, lighter 3/4-ton trucks, and rigs that carry significant cargo. If you're regularly loading the bed or running a rooftop tent, D-rated gives you more margin.
Airing down: Takes a bit more PSI drop than C-rated to get the same flex, but it's manageable.
The heavy hitters. Max PSI of 80. These are the tires you see on 3/4-ton and 1-ton trucks, work trucks, and heavily built overlanders.
Best for: Vehicles over 6,500 lbs, trucks that tow, rigs that carry serious weight.
Airing down: This is where it gets interesting. E-rated sidewalls are thick. Really thick. At 28 PSI, they can still feel like bricks. You're running at 35% of max pressure and the tire still might not be conforming much to terrain. You often need to go lower than you'd expect -- into the low-to-mid 20s -- before E-rated tires start giving you the kind of flex that C-rated tires provide at 20-22 PSI.
Across all load ranges, roughly 30-40% of max PSI gets you good trail flex:
| Load Range | Max PSI | Trail Flex Zone (~30-40%) |
|---|---|---|
| C | 50 PSI | 15-20 PSI |
| D | 65 PSI | 20-26 PSI |
| E | 80 PSI | 24-32 PSI |
These aren't exact prescriptions -- they're starting points. Vehicle weight, terrain, and tire size all factor in. If you're running E-rated tires and wondering why 28 PSI still rides like a lumber wagon, now you know: you haven't dropped far enough relative to that tire's max capacity. Many Rivian owners on E-rated LTs report needing to drop into the low-to-mid 20s to get meaningful trail flex.
Here's a problem most people don't think about: when you swap from SL to LT tires (or change tire sizes within LT), your old door-jamb PSI recommendation is probably wrong.
The door jamb pressure is calibrated for the factory tire. Different tire, different construction, different optimal pressure. Running the wrong street pressure wears your tires unevenly and costs you money.
The chalk test fixes this. Here's how:
Reading the results:
Repeat until you get even wear. Write down the number. That's your new street pressure for these tires.
Do this every time you change tires. SL to LT, LT C-rated to E-rated, same tire in a different size -- any change. It takes ten minutes and saves you hundreds of dollars in uneven tire wear.
You've got new tires. You know the load range. Now what pressure should you actually run on the street?
There's a formula for this. It's not perfect -- it's a starting point that gets you close before you validate with the chalk test.
Starting PSI = (Vehicle Weight / 4 / Tire Max Load) x Max PSI
This assumes roughly equal weight distribution across all four tires (which isn't exactly true, but it's close enough for a starting point). It also uses the linear relationship, which will give you a slightly higher starting pressure than the TRA formula -- which is fine for a starting point you'll validate with the chalk test anyway.
You've got a Ford Excursion that weighs 9,000 lbs loaded. You put on E-rated tires rated for 3,200 lbs at 80 PSI.
Step 1: Average weight per tire = 9,000 / 4 = 2,250 lbs per tire
Step 2: Fraction of max load = 2,250 / 3,200 = 0.703
Step 3: Starting PSI = 0.703 x 80 = ~56 PSI
So you'd start at 56 PSI on the street, then validate with the chalk test and adjust 2-3 PSI at a time until the wear is even.
We've been talking about sidewall construction -- how many plies, how thick, how stiff. But sidewall height also plays a role in how a tire performs when aired down.
Taller sidewalls = more air volume = more flex at any given PSI.
A tire with a 70-series aspect ratio (like a 285/70R17) has a taller sidewall than a 55-series (like a 285/55R22). That extra sidewall height gives you more room for the tire to flex and conform to terrain before the rim gets dangerously close to the ground.
Shorter sidewalls mean less flex, less air volume, and less margin before you're riding on rim. If you're on low-profile tires (55-series or lower), you need to be more conservative with your airing-down targets -- you simply have less sidewall to work with.
This is another reason the factory tires on many modern trucks and SUVs aren't ideal for off-road: manufacturers spec bigger wheels with shorter sidewalls for the look, which reduces the tire's off-road capability before you even get to the SL vs LT question.
For the full deep dive -- including a sidewall height tier table, air volume calculations, and a comparison chart for every popular tire size -- see our sidewall height guide. It covers why 21" and 22" wheel owners face a fundamentally different airing-down equation than trucks on 17s or 18s.
For PSI targets by terrain and vehicle weight, check out the PSI cheat sheet -- it covers specific numbers you can screenshot and save.
Tire choice is a tire choice, not a vehicle choice. Any rig can have SL or LT tires on it. The question is whether the tire you're running can safely support your vehicle's weight at the pressures you want to run on the trail.
Here's the process:
Getting this right means you can air down confidently -- knowing your tires are built for what you're asking them to do. Getting it wrong means running at the margins and hoping for the best. One of those options sounds a lot more like a good day on the trail.
Once you've got the right tires on your rig, a MORRflate AirHub takes the hassle out of actually airing down and airing back up. All four tires at once, matched to the same pressure, in a fraction of the time. Pair it with the MORRflate TenSix compressor for airing back up, and your trailhead routine goes from fifteen minutes of walking circles around the truck to a few minutes of connect-and-go.
Check out the full lineup at morrflate.com.
*More off-road tire fundamentals and gear breakdowns at airdownforwhat.com.*
Picking new tires? The Tire Upgrade Impact Calculator shows exactly how much longer your compressor will run, what the new air-down PSI looks like, and which MORRflate setup keeps up.
Want to learn hands-on?
Reading is great. Practicing with an instructor on a real trail is better. We teach airing down, recovery, and vehicle handling at Sierra Nevada Off Road Academy (SNVORA).