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 three 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 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
Here's what you're looking for:
The first letter(s) tell you the tire category:
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) on a P-Metric tire means it's a beefed-up SL tire -- more capacity than standard, but still not LT construction. Don't confuse XL with LT. They're not the same thing.
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 an SL/P-Metric tire. 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 PSI | Typical Max Load Per Tire |
|---|---|---|
| SL (P-Metric) | 35-36 PSI* | 2,200-2,800 lbs |
| XL (P-Metric) | 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: Always check your specific tire sidewall for the above specs. Some E-rated tires on 16” wheels are rated at 65psi. Some SL tires are rated to 55psi.
Look at that max PSI column. An SL tire tops out at 35-36 PSI. An E-rated LT tire goes to 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 max PSI. Drop the pressure and the load capacity drops right along with it -- proportionally. This is where the math gets real.
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.
Load capacity drops proportionally with pressure. If a tire is rated for 2,800 lbs at 35 PSI (its max), then at 25 PSI, the math looks roughly like this:
(25 / 35) x 2,800 = 2,000 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 250 lbs of margin per tire. Add a couple hundred pounds of gear in the back and uneven weight distribution (the front axle on most trucks and SUVs carries more weight), and some tires are running at 164 lbs of margin or less.
That's razor thin. One good bump, one loaded corner, and you're over the rated capacity of the tire.
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.
A C-rated LT tire rated at 2,800 lbs at 50 PSI, aired down to 25 PSI:
(25 / 50) x 2,800 = 1,400 lbs per tire
Wait -- that's actually lower than the SL example above. So what gives?
The answer is that 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:
(25 / 80) x 3,500 = 1,094 lbs per tire
That's not great either. But here's the thing -- the structural sidewall of that E-rated tire is physically stronger than the math suggests. The ply construction supports loads beyond what the proportional calculation implies, especially in the lower pressure range. 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 P-Metric tires. Not LT. P-Metric.
This is true for the stock Pirelli Scorpions and the stock Goodyear Wranglers. Check the sidewall: you'll see a "P" prefix or no prefix at all (Euro-metric). Either way, it's SL-class construction.
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 stock tires are rated for their max load at around 35-36 PSI. The door jamb says to run 48 PSI for full load. That's already above the tire's max-pressure load rating -- Rivian gets away with it because the XL-rated variants allow slightly higher pressures.
Now air down to 25 PSI for trail driving.
Your load capacity per tire at that reduced pressure is pushing right up against the vehicle's actual corner weight. On a heavy R1T with gear, you may be exceeding the proportional load rating at that PSI. The tires will technically still hold air and roll, but you're operating outside the designed safety margin.
This is why you see so many Rivian owners swapping to LT tires -- BFGoodrich KO2s, Falken Wildpeak AT4Ws, Toyo Open Country AT3 EVs. It's not just about tread pattern or looks. It's about getting sidewall construction and load capacity numbers that actually support the vehicle's weight at trail pressures.
This isn't a Rivian-specific issue. Any heavy vehicle on SL 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 P-Metric tires.
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 25 PSI, they feel like bricks. You're running at roughly 31% 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 high teens -- before E-rated tires start giving you the kind of flex that C-rated tires provide at 22-24 PSI.
Across all load ranges, roughly 35-40% of max PSI gets you good trail flex:
| Load Range | Max PSI | Trail Flex Zone |
|---|---|---|
| C | 50 PSI | 17-20 PSI |
| D | 65 PSI | 22-26 PSI |
| E | 80 PSI | 28-32 PSI |
These aren't exact prescriptions -- they're starting points. Vehicle weight, terrain, and tire size all factor in. But if you're running E-rated tires and wondering why 25 PSI still rides like a lumber wagon, now you know: you haven't dropped far enough relative to that tire's max capacity.
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).
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.
Rivian R1S at 7,200 lbs on LT C-rated tires rated for 2,800 lbs at 50 PSI.
Step 1: 7,200 / 4 = 1,800 lbs per tire
Step 2: 1,800 / 2,800 = 0.643
Step 3: 0.643 x 50 = ~32 PSI
Start at 32, chalk test, adjust. This is why many Rivian owners on C-rated LTs report running in the low-to-mid 30s on the highway -- the math backs it up.
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 breakdown on 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.
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).