How To Read A Tire Sidewall

Last Updated: 2/26/2025

Time to read: 11 minutes

HOW TO READ Your Trailer Tire Sidewall

And Pick Your Next Set Of Trailer Tires

Your old trailer tires have taken you down countless highways, backroads, gravel roads, and trails. They’ve hauled in rain, in snow, in sweltering heat, and it’s really starting to show. Let’s face it, the time has come to get yourself some new tires. If you’ve never needed to purchase trailer tires before, this might sound like a bit of a daunting task, but it’s really not too complicated. A lot of the information you’ll need to know can be found right on a tire’s sidewall. We’re gonna break down the different specs you’ll find listed on a tire, explain just what they all mean, and go over why they matter. And by the end of this read, you’ll be ready to choose your very own trailer tires, without having to ask some salesman to figure it out for you. So prepare yourself: you’re about to earn your very own degree in tire-ology.

How to Read Your Tire Sidewall

Tire Size - It’s Not Just Small, Medium, Large

The easiest way to replace your old trailer tires is to just look at their sidewalls and find new tires with identical specs. However, if your old tires didn’t work well with your trailer, or you just want to look for something a little different, then starting from scratch is probably the better option. And in a situation like that, you’ll definitely need to know how to understand a trailer tire’s size.

A tire’s size is a lot more than just how big the tire is (although that is part of it). Check out the sidewall of any tire, and you’ll see a string of letters and numbers. THIS is your tire’s size. It indicates information about the specs of a particular tire, and it’s got all the info you’ll need to understand if you want to pick out your own set of tires.

One big thing you need to remember when it comes to tire size is uniformity. It’s very important to only use tires that are all uniform in size. If you use differently sized tires, there will be uneven load distribution on your trailer, and your tires won’t wear evenly. Eventually, this can lead to equipment damage and tire blowouts, so it’s just not worth the risk.

While you do have some choices when you pick out new tires, you shouldn't just throw any old set of tires on your trailer without close inspection. A tire's size tells you a lot about the sort of conditions where it'll work best (on vs. off-road, heavy loads vs light loads, high speed vs low speed, etc.). Really considering all the qualities of a tire is key to finding the right fit for you.

Metric vs. Numeric Tire Sizes

There are two main sizing-styles that you might see on trailer tires today: Metric (the most common for trailer tires), and Numeric (usually only on the smallest trailer tires). A tire with a metric-style size will usually have the following 7 pieces of information (depending on the specific tire) listed on its sidewall:

Application Type -that’s the general kind of tire (P, LT, ST, etc.)
Section WIdth -the width from sidewall to sidewall
Aspect Ratio -the ratio of section height to section width
Internal Construction -whether the tire is radial or bias
Wheel Diameter -the diameter of the wheel that tire will ideally be mounted on
Load Rating -either load range, which is an approximation of the tire’s strength, or load index, which tells you a tire’s carrying capacity
Speed Rating -the maximum speed your tires can safely go

*It’s worth noting that not all tire manufacturers include EVERY ONE of these values in their sizing, some (like the example image to the left) only list 5 or 6 of them.*

Because the majority of trailer tires are metric, we’re going to break down all the specs in the order you'd see them on a metric tire. However, that doesn’t mean that metric tires are the only trailer tires you should know how to read

Numerically sized tires aren’t as popular as metric, but you can still find them in ust about any tire shop. If you have a trailer that rides lower to the ground and needs smaller tires, you’ll definitely want to know how to read numeric sizing. Now, this style of sizing does look pretty different from metric, but it still conveys some of the same kinds of information. Numeric sizing is usually indicated in one of two ways, varying from tire to tire. You’ll either see:

Section Width, Wheel Diameter, and Load Rating, or
Section Width and Wheel Diameter alone.

As you can see, numeric sizing isn’t too complicated. Once you understand how to make sense of the specs on metric tires, you’ll be well on your way to reading numerically sized tires as well.

Example of a metric tire

Example of a numeric tire

Application Type (ST vs LT vs P Tires)

The first letters in your tire’s size indicate the tire’s application type. When shopping for tires, you’ll see a couple different application types, (for example, P for passenger tires, LT for light truck tires, etc). But when it comes to trailer tires, there’s only one kind you need to worry about: ST.

“ST” stands for “Special Trailer,” and that is the only kind of tire you should ever put on a trailer. No other kind of tire is built to withstand all the heavy use and abuse that a trailer tire sees on the road. Putting a tire that isn’t marked “ST” on your trailer is asking for a blowout.

You might be wondering why that’s the case, considering that some truck and passenger tires are built to be fairly strong. That’s because in order for any tire to have a cushy enough ride to be comfortable, it has to give up some toughness and sidewall rigidity. Otherwise every passenger would leave the vehicle with a very sore backside. Now, less rugged tires are alright for a truck or passenger vehicle, but when it comes to a trailer that’s going to be carrying thousands of pounds of payload, you need all the strength and rigidity you can get. Not to mention, it doesn’t matter if a trailer tire gives you a cushy ride, because hopefully nobody is riding in the trailer.

ST tires are made to maximize strength, load bearing ability, durability, and performance under rough conditions. They’re built to get the job done, and that’s why they’re the only tires you should ever consider using for your trailer.

Section Width

Section width comes after application type in a tire’s size, and it isn’t as simple as the overall width of the tire. It’s actually the width of a tire from the widest part of its outer sidewall to the widest part of the inner sidewall in millimeters—not accounting for any raised lettering or decorations, or protective ribbing that might be found on the tire. Section width is measured once a wheel has been mounted on the correct size rim and has been fully inflated, but not loaded down with any weight. Using a tire with the wrong section width can mess up steering response, traction on the road, and just make for a dangerous driving experience, so it’s important to take it into account.

Pictured: Metric tire on the left, numeric tire on the right

Aspect Ratio

Aspect ratio comes after section width. It’s basically the ratio of a tire’s section height (the distance from the wheel rim to the top of a tire’s tread) to its section width. Aspect ratio affects steering response and grip, as well as rolling resistance, so it’s important to make sure you have the right aspect ratio for your trailer’s needs.

Internal Construction (Radial or Bias)

There are two main types of ST tires, Radial Construction (represented by a R), and Bias/Diagonal Construction (represented by a D).

Radial

Radial tires are made from rubber plies that radiate out at 90 degrees from the center of the wheel. Radial construction means that the tires have a flatter tread that wears evenly over time, and they don’t get flat spots when they’re left parked for long periods. Thanks to both of these factors, radial tires have a fairly long lifespan. They also have relatively stiff sidewalls, which allow them to turn corners and maneuver really precisely. Radial tires also dissipate heat very efficiently and have a smoother ride, so they’re good for long drives on the highway (and overall great for recreational trailers).

It’s worth noting that these tires have a “lower lateral stiffness,” which can cause them to sway during high-speed travel. They also tend to run a little more expensive than bias tires, so they may be a better option for someone who plans to tow frequently, who wants to really invest in good tires.

Bias

Bias tires are built from rubber plies that crisscross at an angle. While they’re less common nowadays than their radial brothers, bias tires still have a number of advantages. These tires have plies that crisscross all the way around, which means their sidewalls and crowns (the part of the tire that touches the road) are interwoven. Because of this, they tend to have super thick and strong sidewalls, which are great at resisting punctures and slices. The crisscross design also makes bias tires more flexible than radial tires, so they can be a good choice for traveling on rough terrain and bearing heavy loads (so they’re great candidates for agricultural trailers).

Bias tires tend to be a little more affordable than radial tires, so they might be better for someone who tows infrequently, or who’s working on a tighter budget. However, they are prone to developing flat spots from sitting in place, which can cause performance issues on the road. Their flexibility can also be a double edged sword. Because they’re ultra flexible, they tend to squish on sharp turns, which makes it harder for them to turn with much precision. Bias tires are also less efficient at dissipating heat, making them more likely to overheat and suffer blowouts. All this considered, they can still be an excellent option for the right towing setup.

Pictured: bias tire (left) and radial tire (right)

Wheel Diameter

Wheel diameter is the distance across the wheel’s face at the widest point, excluding the outer lip/edge of the wheel. It’s measured in inches, and shown after construction type in the tire’s size. Wheel diameter is mainly important for making sure you have the right size tire for a certain wheel. Having the wrong tire can mess with steering sensitivity, hurt the ability to brake correctly, cause premature damage and wear to the tires, and generally make your trailer handle poorly.

Load Rating

After the wheel diameter, you should find a load rating in your trailer tire’s size. A load rating, generally speaking, is a way to indicate the strength of a tire. There are two main measurements that can be used to do this: Load Range and Load Index. While they sound similar, they’re not the same. Generally speaking, you’ll be more likely to see load range on ST tires, and load index on P-Metric (passenger) tires. If you want to learn more about load range vs load index, check out our article comparing them here. For the purposes of this article, we’re just going to go over load range.

Load range is a more general way of indicating the overall strength of a tire. It’s based on an older type of measurement, the “ply rating,” that measured a tire’s strength by how many plies thick it was. Because modern tires don’t need as many plies of rubber to give them the same amount of strength, load range was developed as a way to approximate how strong a modern tire is at it’s ideal air pressure by comparing it to an old-style tire with the same “ply rating.” If you’re confused, think about how we still measure engine strength in horsepower, then stop thinking about it and just move on because you’ll still be confused.

Load range is represented by a letter that corresponds to a specific ply rating, and the further along in the alphabet the letter, the stronger the tire. While load range isn’t a super precise or uniform measurement, it’s worth mentioning that the most common load ranges for trailer tires are C, D, and E rated tires. Always remember, since trailer tires are meant to be fully inflated to their max allowable air pressure, load range is only accurate when a trailer tire is fully inflated.

No matter what load range is listed on your trailer tire, you should always be careful not to overload your tires with too much cargo. When you’re outfitting your trailer, always consider your GTWR (Gross Trailer Weight Rating). The GTWR is a number that tells you how much weight a trailer can support (including the weight of the trailer itself, and any cargo that might get loaded onto it). Your trailer is only ever as strong as its weakest component, so it’s very important to pick tires that can match the strength of your trailer. In fact, it’s generally good practice to look for tires that can support just slightly more than your trailer’s GTWR, so you have a bit of a buffer if your trailer ever happens to be overloaded. *Just remember, having stronger tires won’t increase your trailer’s GTWR, it’ll just make your tires less likely to blow out.* For more on this subject, check out this article on making sure your trailer isn’t overloaded.

Speed Rating

A tire’s speed rating essentially tells you the maximum speed your tire can go while maintaining safe driving conditions and good tire performance. Speed rating is only accurate when a tire is inflated to the correct tire pressure (for trailer tires, that’s always their maximum air pressure), and it’s loaded at/ below the max carrying capacity. Speed rating is indicated by a single letter, A through Z, and it usually increases with a tire’s strength. It’s important to mention, not every metric trailer tire will have a speed rating on it's sidewall. If your tire doesn't have a speed rating listed, just remember that the standard speed rating for a trailer tire is 65 mph.

Although you may feel comfortable driving above the speed rating with a loaded trailer, that doesn’t mean you should. If you don’t want your tires to pop like balloons while you’re hauling thousands of pounds of cargo on the highway, please obey your tire’s speed rating. Or don’t, and learn firsthand just how high trailer repair prices are these days.

Bonus Section!!! How To Tell A Tire’s Age

If you’re curious about how to tell how old a trailer tire is, there’s a handy little number on the sidewall that will give you exactly the info you’re looking for! For all tire’s manufactured after the year 2000, the DOT requires that the age of a tire be displayed on its sidewall. To find this info, just look for a smaller line of letters and numbers that starts with DOT. At the very end of that line will be a 4 digit code that tells you the week and year that tire was manufactured. For example, if the code reads 1822, that means the tire was manufactured in the 18th week of the year 2022. This can be handy info to have, since trailer tires wear out and need replacing every 3-5 years (or every 10k-12k miles). For more on this topic and other common questions about trailer tires, check out our article here!

The tire above was manufactured during the 17th week of 2018

So How Does All This Help You Choose Your Own Tires?

It’s pretty nice to be able to pick out the tires you know will work best, without having to consult somebody that’s never towed a day in your vehicle or with your trailer, whose advice might not be very personal. That said, it’s also a significant undertaking to pick out your own tires. You have to make sure that you trust your own expert advice before you go into the tire shop and pick out what you want, and have faith that that choice won’t leave you stranded on the shoulder of a 4 lane highway, or on some old gravel road in the middle of nowhere. Keeping that in mind, it's key to make sure you always consult the owner's manuals for your trailer and towing vehicle when making a choice like this. They’re likely to have some excellent information that can speed up the decision making process. But other than that, I have nothing else to teach you! Congratulations, you now have your very own degree in tire-ology, be sure to use it wisely. Happy towing!