I’m not an expert in this space by any means, but I’ve been forced to learn a bit about it over the past 7 years of being a camper owner. I’d like to share what I’ve learned, and to learn from the experience from all of you as well. Drop your knowledge in the comments, and let’s all get smarter together.
As always, when talking about matters of weight and campers, you’re gonna have two tribes. One tribe — and this tribe definitely has the lawyers and the cops and the engineers and all sorts of other smart people on their side, so mess with them at your peril — will tell you this whole post is unnecessary, just make sure that you replace the tires with the exact tire specified on your door placard, and never exceed the maximum payload on that placard, and you won’t have to ever worry about stuff like this. That tribe is RIGHT, and if this paragraph resonates with you, you might just wanna stop reading here.
The other tribe looks at these matters as a little more flexible. Specifications as more of a suggested starting point for upgrades, weight limits as points at which you might start to become concerned. There are lots of legal, maintenance, performance, and other liabilities associated with being a member of this tribe. If you’re comfortable with assuming some of those risks, or if you just wanna learn a little bit about tires just in case, read on.
My take is this group is roughly split down the middle between the two tribes. Many folks in this group load up their trucks a little (or a lot) heavy. The very first thing you need to be concerned about if this describes you, is the safety of your tires. How can you know if your tires are at risk, and what can you do to reduce the risks?
Tires are conceptually pretty simple. There’s a airtight balloon, with rubber on the outside to stick to the ground, built around a wheel or rim. The rubber holds the air in. The air pressure is what actually bears the load of your entire truck and everything in or on it. The air and tire transmit that load to the wheel, which in turn transmits it to the axles and so on. But note that the air is the main thing in the tire that holds the load. If you have more air pressure, you can handle more load.
Tires used to be constructed from a fabric wound around a circular form and encased in vulcanized rubber. The warp and weft of the fabric would be at different angles, or biases, in each layer, or ply. These old tires were called “bias-ply tires” as a result, and a tire with more layers of fabric could handle more air pressure and thus more load. Tire stores would advertise truck tires as “10-ply”, “12-ply”, and so on. More plies, more load capacity. If you’ve ever heard of someone say a set of tires was so worn out that “the cords were showing”, they’re talking about the coarse threads that made up that fabric being exposed by ablated rubber.
Those old bias-ply tires kind of sucked. The fabric just didn’t offer as much stiffness to the tire as you might want, and so the tire felt greasy and squishy as a result. These days, almost every tire you find will be a “steel-belted radial”, which means the structure of the tire will be formed from a wire or cable of steel, formed radially (which is a fancy way to say in a circle), encased in rubber. The exact layup of the steel wire is specific to each tire, and probably a big trade secret, so obviously the old “number of layers” rating won’t really apply anymore. Instead, today’s tires are accompanied by a “load range” which indicates how much air pressure the tire can reasonably withstand. Remember, more air means more load.
The load range is in the tire size specification on the side of every tire. Fully breaking down that tire size is beyond the scope of this article, but as an example, let’s just look at the tires that are on my 2000 Ford F-350 non-dually truck:
LT265/75R16 123/120R E
This means this is a LT (light truck) tire, with a 265mm tread width, and a sidewall height that is 75% of the tread width (or about 199mm high), on a 16” rim. Don’t get me started on the mixing of metric and imperial units, it’s just how they do it. For the purposes of this article though, we care about the last two bits of that string of text: 123/120 is the load index, sometimes also called the service description, R is the speed rating (specified as a letter that indicates a do-not-exceed speed for the tire or it might start blowing itself apart — in this case 106mph, which we can safely say my camper has never experienced) and the E is the load range. We’ll cover load range and load index in detail.
Load range and load index, and their relation to PSI
The load range on your tire is indicated by a letter (A through G are the ones I’m most familiar with although I’m sure the tractor-trailer guys will tell me it’s just the beginning). The load range indicates how much air pressure the tire can reliably hold. For example, a load-range “E” tire, standard on most of the trucks you all use to carry your campers, can hold a maximum of 80 pounds-per-square-inch, or PSI, of air pressure. Not every tire uses this kind of rating, but most used by our trucks do. If you want to see what the different load ranges are, and their corresponding max PSI rating, check out this article (I have no affiliation with Tire Rack but I have spent a bunch of money there!): https://bit.ly/3kzuJTM
Next, the load index. On my tire, this is specified as a dual range: 123/120. The first number is how much weight you can put on the tire in a SRW configuration. The second is if you use the tire on a dually. The lower index number for a dually is so that if one of the tires on one side of the axle blows, its pair won’t immediately self-destruct as well. The load index specifies how much load that tire can take, but ONLY AT MAXIMUM PSI. If the tire is inflated below the maximum specified by the load range, it will handle somewhat less load. I haven’t seen a chart that will show how much less load is possible for less air, but if you know of one let me know. On my tire, that maximum load, at 80 PSI, is 3417 lbs. This number is helpfully printed on the sidewall of the tire too, along with the maximum PSI. Note that the load is per-tire — so my truck’s tires can carry a maximum of 6834 lbs of truck and camper and people and trailer weight and everything else, total. In a dually, we distribute that weight across four tires, but at the lower load index of 120 (3086 lbs), so 3086 x 4 = 12344 lbs total. This is the main, but not the only, reason duallys are recommended for big heavy campers.
If you’ve been following along up to this point, you might ask — if a more-inflated tire can handle more load, why don’t we just always load up tires to the max PSI? The reason is that while the tire will handle more load, it might not need to, and an overinflated tire for the load you’re carrying will be stiff, bouncy, and will wear in the center more than in the edges. It’s good for the PSI to match the load on the tire. This is also the reason that the same tire on a front axle will often have a lower PSI specification on the door placard than one on a rear axle — trucks are designed to carry more load on the rear tires. A truck camper will put the majority of its weight on the rear axle, so one way to carry a little more load than your truck is rated for is to simply air up the tire to its max.
A tire will heat up under use. According to Boyle’s law, which you might remember from high-school physics if you were paying attention, pressure increases with temperature. So it’s common for a tirewarmed by use to be 5 PSI higher than the same tire with the same amount of air in it, but cold. It’s unclear to me whether the max PSI stamped on the tire is the true “do not exceed” PSI, or the max cold PSI. Maybe someone in the comments below will know.
How much air should I put in my tires?
Simple answer is, fill the tires to the factory specifications on the door placard, don’t exceed the max payload rating, and you’re good.
A more complete answer is, you should put enough air in the tires to support the load that you’re carrying. The only way to know how much load you’re carrying is to hit a truck scale. You can get an idea from going to the dump and weighing each axle separately, but if you have one anywhere near your house, you should pay the $12 for a reading on a CAT truck scale. This will tell you exactly how much weight is on each axle, as well as any towing axles you may have. Note, too, that if you’re towing, 10–15% of your trailer weight much be borne on the hitch, which is in turn carried by that rear axle.
Most of the weight of the camper will be on the rear tires, and most of the weight of the truck (the engine, transmission, front suspension, transfer case, front axles, etc) will be on the front axles. Overall, this means the rear tires need more air than the front if you’re carrying a heavy load. But if you have a big cabover section, or a front hitch receiver rack, then you will need more air in the front tires. How much is sort of a matter of opinion, unless someone can produce a load/PSI chart in the comments. But many guys when carrying a heavy load will air up above the factory specs in front. I recommend that you look at your scale reading, and if you’re much heavier with the camper than without it in front, add some air. If you notice that the tire is overly bouncy, or hard, or is wearing more in the middle than the edges, let some out.
What can go wrong if I overload my tires?
Pretty simple — overloaded tires can blow out at speed while you’re carrying the top-heavy load of your camper and maybe towing as well. This happened to me four years ago on a long road trip with my family. The tire completely exploded, the rim was destroyed, and the exiting tire wrecked some sheet metal on the way out. Thankfully, that was all that happened; we didn’t roll over or lose control. I credit the heavy trailer with being a little bit of a boat anchor that helped slow us gradually. It could have been much, much worse. I don’t actually know if this was because we were overloaded (I hadn’t yet used a scale) or because the tire was old and damaged, but I suspect both factors were at play. It was a close call.
You might get some hints that you’re running too heavy BEFORE you have a blowout. If you experience these warnings, heed them immediately. The sidewall can be ballooned out. It’s sort of normal for a sidewall to have a little warping in and out, but a big outward protrusion is bad news. Similarly, the top of the tire, at the tread, could have a high crown in the middle. The tire should be more or less flat across the tread, and if it’s sticking way up in the middle, a belt has broken inside the tire. You might experience a vibration that increases with vehicle speed if this happens. If you experience any of these things, DON’T IGNORE THEM. Get out. Look at your tires for these signs above. Feel them. If one is way hotter than the rest, this indicates danger. Check their PSI. If anything worries you, stop, and swap the tire for your spare. And make sure your spare has the right amount of air to be used as a rear tire; that’s the one that is most likely to have problems, and it’s easier to let some out and mount it on the front than to add air on the side of the road.
What do to if your tires are overloaded
The safest answer is, get a bigger truck. Tires aren’t the only consideration in carrying heavier load. Following the load-bearing chain from the tires in: the wheels, the wheel studs, the axles, the axle housing, the rear end, the shocks, the leaf springs, and the frame all have their own maximum load ratings. Conventional wisdom states that the tire will be the weakest link in the chain, but if you upgrade your tires, you might hit the limits on one of these items instead. And the brakes will be less effective with more load too. Each of these items could be upgraded, but at some point smart money would say to just buy a bigger truck.
But if you’ve done the research and feel confident that the tires are holding you back from (more) safely carrying that huge camper and boat you want, it’s possible to upgrade, a little. Most “light” truck tires (everything smaller than a class 4 truck, so that’s F350/3500 and lower) come in a maximum of load range E. And the wheels that came with the truck won’t be rated much higher than the tire is; that’s just good engineering to not spend more weight or money on the wheel than is indicated by the design specifications. But it’s possible to replace your wheels with 19.5” wheels, which is the standard for commercial medium-and-heavy-duty trucks, and this unlocks the possibility to run much heavier commercial tires, up to load range G. It’s important to ensure that you have the same, or at least similar, overall tire diameter to your stock tires, or your final drive ratio and speedometer calibration will have changed. Both of these things can be fixed with more upgrades and changes, but again this is more money, and at some point you will wish you just had more truck. Search the internet for 19.5” wheels and tires for your truck, and you’ll probably find options.
Phew! That was a long article. Thanks for reading all this way, people don’t always do that on the internet. If there’s something I got wrong, or something I missed, please help us all get smarter by putting it in the comments below. If your comment is basically “overloaded trucks are Satan” or “weight police are weenies”, please just don’t bother commenting at all — you’re not going to move many people to your tribe that way. But I hope that you learned something from this, and that you’ll maybe teach me something too.
Here’s an overview of what you learned:
- tire loading is determined by the relationship between load range, load index, and PSI
- you need to know how much your truck actually weighs if you’re going to make good decisions
- overloaded tires can and will cause accidents and might kill you or someone else
- it’s possible to upgrade tires to increase design limits, but it’s not always a great idea