Critical Information for Choosing Trailer Tires
In This Article:
The Ultimate Guide for Choosing the Right Trailer Tires All the Details You Need, and None You Don’t
It All Starts with the Size: Trailer Tire Sizes Explained
The Ply Rating, Also Called Load Range, Is a Measure of Toughness
What Is Section Width, and Why Is It Important?
The Pressure, Oh the Pressure! PSI Matters
Getting Up to Speed on Speed Ratings
Of Course, the Outside Tire Diameter Is Important
Why You Should Pay Attention to Load Capacity
Radial vs Bias—Does it Matter?
Tread Depth Is Important Too
Why Tire and Wheel Packages Can Mean All Kinds of Savings
Look for Fresh Trailer Tires
When You Need Trailer Tires, Count on Treadworld
The Ultimate Guide for Choosing the Right Trailer Tires
All the Details You Need, and None You Don’t
Let’s face it. When it comes to getting the best trailer tires for your pull behind trailer, there’s a ton of confusion. Much of it is helpful, some of it worthless, a lot of it is contradictory. The tire experts here at Treadworld, home of the finest trailer tires anywhere, think choosing the best tires for your boat trailer, tool trailer, travel trailer, toy hauler, RV trailer, livestock trailer, any tow behind trailer, should be a simple matter of determining the right fit, and knowing what job you need it to do, then making your selection. We’re here to cut through the B.S., and identify the most critical information for choosing a trailer tire, and tell you why that information is important to your choice—size, ply/load rating, section width, PSI, speed rating, outside diameter, load capacity (single/dual) and tread depth. Plus, we’ll take a quick look at the advantages of bias tires versus the advantages of radial tires, and offer some quick thoughts on why it’s important to get tires that are not just unused, but fresh.
It All Starts with the Size: Trailer Tire Sizes Explained
If you know the size tire you need, making your selection at Treadworld is a piece of cake: go to the trailer tires page, click on the size you want and choose from the options shown. But in an industry where it sometimes seems that making the simple complicated is normal, it’s not really that easy, and sometimes even figuring out the tire size you need can be a challenge. That’s because how the industry identifies tire sizes has changed over the years, and today there is something of a mash of three different tire size identification systems: numeric, alpha numeric and metric.
To make more sense of tire sizes, it’s good to understand aspect ratio. The aspect ratio is a percentage identifying the height of the tire’s sidewall (from wheel to tread) as a percentage of the tire’s section width (the widest point of the tire measuring sidewall to sidewall, see below). If a tire has an aspect ratio of 78, it means the tire’s height off the rim is 78% of its section width. This is important as a comparison tool because we know that higher aspect ratios mean larger sidewalls, which favor a smoother haul and a heavier load.
The numeric system is typically found on smaller trailer tires and consists of two or three numbers. For instance, 4.80-8 indicates the tire section width is 4.80 inches and the diameter of the rim the tire will fit is 8 inches. 16.5×6.50-8 indicates the height of the tire is 16.5 inches, the height of the sidewall is 6.50 inches, and the diameter of the rim is 8 inches.
The alpha-numeric system is an older naming convention commonly used on 13-inch to 15-inch trailer tires. For example, with an H78-13 tire, the H stands for a load rating (which we’ll talk about more below), the 78 is the aspect ratio and the 15 indicates the applicable rim size. Most alpha-numeric names have been converted to a metric naming convention (see next paragraph). In this case, the metric equivalent for H78-15 is 225/75D15.
The metric system is being phased in by trailer tire manufacturers. For instance, ST225/75R15 is a common trailer tire size. The ST stands for special trailer and tells you the tire is designed with stiffer sidewalls to hold up to use on a trailer. 225 is the width of the tread in millimeters, 75 is a ratio representing the height of the sidewall as a percentage of the tread width (the aspect ratio), and 15 is the size of the rim this tire will fit. In inches, the tire is roughly 9 inches wide, 28 inches tall and mounts on a 15-inch rim.
Typically, tire sizes are listed on the sidewall of the tire. This is true for RubberMaster Trailer Tires.
The Ply Rating, Also Called the Load Range, Is a Measure of Toughness
The trailer tire’s load range is a relative measure of how tough it is and how much it can carry, at a specific air inflation level. Load ranges are a comparative value of a tire’s strength and are listed on the tire (letters B, C, D, E, etc.). The higher the load range rating, the more durable the tire and the more weight it can safely carry. This measurement is actually an offshoot of an older measurement known as ply rating, which was relevant when load-carrying capacity was determined by the number of plies—reinforcing belts added to the tire. That’s no longer a good indicator of tire strength because, thanks to advances in tire manufacturing, today’s tire construction uses fewer, stronger plies—therefore, load range defines the tire’s toughness and load capacity, rather than specific information (i.e., number of plies) about its composition.
So, today’s “E” load range tire may need only two corded plies instead of ten, to reach the “E” level of toughness and durability. You’ll find the load range on the sidewall of the trailer tire expressed as either a letter (A-G) or a number of plies, ranging from A-2 plies to G-14 plies. Since load range/ply rating is determined based on the construction of the tire, the same load range/ply rating differs between tire sizes, so a load range/ply rating D in one size will have a different weight-carrying ability than a different tire size with the same range/rating.
Complicating the matter, you will often see terms used interchangeably because tire experts use different classifications to describe a tire’s ability to carry weight at a certain air pressure. Along with ply rating/load range, you will see “max load rating”, “carrying capacity” and “tire load index” all used to denote how much weight a tire can support. The ply rating/load range that we’re talking about here provides a comparative idea of the weight the tire can carry and its relative durability, but is different from those three terms mentioned above (see below).
What Is Section Width, and Why Is It Important?
The section width helps you determine the proper tire size that can fit on your wheel. In short, a trailer tire’s section width is the distance between the widest points of the tire’s outer and inner sidewalls, when the tire is properly mounted and fully inflated, without a load. It’s a necessary measurement to consider when you’re choosing the best trailer tires because your tire needs to fit your rim—an improper fit will affect handling and stability—and it also needs to fit within the suspension components, fender wells and overall design of your trailer. Typically, wider tires will carry more load.
Wider tires also require more power to pull because they offer a wider footprint and therefore have more road resistance, which means a lower miles per gallon of gas average. With wider tires, hydroplaning on wet, slippery surfaces is a larger consideration, but they will typically provide better grip on dry surfaces. On the other hand, narrower width tires will provide surer traction in wet conditions, but are only suited for lighter loads. Wider tires have been shown to help alleviate trailer sway. The section width is noted on the numbers you’ll find on most every tire.
The Pressure, Oh the Pressure! PSI Matters
PSI, pounds per square inch, is a very important measurement for trailer tires. After all, it’s not the tire that carries your load smoothly down the road, it’s the air inside the tire. The higher the PSI, the heavier the load the tire can safely carry and the less heat that will be generated. Not that you want to over-inflate your tires—that’s a whole different Pandora’s Box you don’t want to open.
Tires are rated for optimal PSI, and you will be best served when your tires are inflated to their max PSI in order to take advantage of their full carrying capacity, but also keep heat generation at a minimum. Less than optimal PSI means more flex in the sidewall, which means more heat is generated—which at best leads to more wear on the tire and at worst leads to failure and blowout—and lower mileage per gallon. With low trailer tire air pressures, you’ll require longer braking distances, and handling can be compromised, none of which is good when a sudden situation change requires quick braking.
It’s also good to know that PSI changes with temperature. The rule of thumb is that for every temperature increase of 10 degrees, the air pressure in your tires will increase by approximately one PSI, and vice versa in colder temperatures. While increases/decreases of one PSI may not sound like much, it can have an impact. If you start out in the cool of the morning, then drive through the heat of the day, your change in PSI could be as much as 5-10. Over time the result can impact performance, treadwear and fuel efficiency.
Getting Up to Speed on Speed Ratings
Speed ratings for tires provide you with the manufacturer’s maximum recommended speed the tires can function at properly while carrying their load safely, in ideal conditions. Speed ratings are safety ratings—that provide you with a limit you want to stay under in order to stave off negatives, like an increase in inflation pressure, a heat build-up and ultimately failure of the tire. You see, when a tire has massive centripetal acceleration forces acting on the rubber when it’s rolling, those forces are held in check by the tire construction, including the reinforcing belts (internal plies). When speeds exceed the tire’s capability for lengths of time, those plies can break down, increasing the likelihood of a tire failure. Additionally, excess speed increases heat in the rubber due to the tire flexing more quickly, which can result in the rubber breaking down, which leads at best to shaking or wobbling, and at worst, to a blowout.
Tire speed ratings originated in Europe in the 1960s with three ratings: S, H, and V. These days, across all types of tires and vehicles, there are over 20 ratings, each designated with a specific letter of the alphabet. J, for instance is used for tires rated up to 62 miles per hour. M indicates a speed rating of 81 mph. For a long time, trailer tires were only rated at 55 miles per hour, for the most part because of the differences in construction versus passenger tires. Today, trailer tires that don’t have a specific rating are assumed to be rated at 65 miles per hour, but thanks to advanced technology in manufacturing, many ST trailer tires today are speed rated as high as “M,” which represents 81 miles per hour.
Of Course, the Outside Tire Diameter Is Important
As you would expect, the outside diameter of your trailer tire is the measurement of the height of the fully inflated tire from the road to the top of the tread. No surprise, the diameter of your trailer tires is another measurement that is critical to tire selection because it tells you whether the tire will fit on the trailer within the clearance constrictions created by the trailer’s design and frame. Larger trailer tires will carry more weight, and in theory, will provide a smoother ride—which is certainly true on paper, but the reality is that you likely won’t feel much difference with your trailer. Typically, the tire diameter measurement can be found on the sidewall, following the “R” (which stands for radial), for example. While it is sometimes permissible to go up a size, generally, your safest best is to replace your trailer tires with the same size as the trailer was equipped with when you got it.
Why You Should Pay Attention to Load Capacity
Simply, a trailer tire’s load capacity explicitly indicates the maximum weight recommended for the tire. This varies by tire, and by inflation pressure—a larger tire can hold more air, which in turn gives the tire a higher weight capacity. You might think that to estimate the capacity of your trailer, you could multiply the load capacity of the tires by the number of tires, but it likely won’t surprise you that it’s not quite that easy. You also have to take into consideration the gross axle weight rating (GAWR), which tells you how much the trailer can carry, which includes the static weight of the trailer plus the maximum weight of cargo. Once you know the GAWR, you can select tires that match the trailer capacity, or best case, are rated a little higher. The prudent person will demand 10% to 15% extra capacity in the tire ratings for safety because, despite the best of intentions, trailers are often overloaded accidentally, or can suffer due to the dynamics of the road and conditions, taking a tight corner at higher-than-prudent speed, for instance.
Here at Treadworld, we provide you with two weight capacities for trailer tires, “single” and “dual.” Trailers that have tandem axles, and therefore use eight tires instead of four, will often have a lower per-tire rating because, in dual-tire applications, the load on each of the tires in a pair is never exactly the same, and the path each tire travels is never precisely the same, which can put more of the axle load on the inner tires or cause more wear on the outer tires. Industry standard practice is to rate dual axle tires 10% -15% lower, for safety’s sake.
Most Treadworld Trailer Tires will arrive with a tread depth of 12/32nds of an inch. To put that in perspective, the U.S. Department of Transportation requires semi-trailer tires to have a tread depth of at least 2/32nds, and many consider 6/32nds to be a good tire tread depth. The reason tread depth is important is because it can affect the distances it takes you to stop, especially in wet and snowy conditions.
Radial vs Bias—Does it Matter?
When selecting trailer tires, you’ll have to decide between two types of tire construction, bias ply and radial ply. The difference lies in the placement of the reinforcing belts. A bias ply tire is sometimes referred to as cross ply tire because that’s the nature of the construction—body ply cords extending diagonally at a 30 to 45-degree angle to the direction of travel from bead to bead (the edge of the tire that sits on the wheel) with successive plies laid to form a crisscross pattern. These cords, designed to enhance the strength of the tire, are typically a combination of polyester and steel. With radial tires, these cords run at a 90-degree angle, or across the tire, to the direction of travel.
You might think it’s a distinction without a difference, but you’d be wrong. The construction of radial ply tires typically makes them more flexible, leading to better traction, stability, and treadwear. They also tend to run cooler, which can provide an advantage in terms of longevity. However, they are not great in harsh environments. On the other hand, the construction of bias ply tires tends to give them stiffer sidewalls, which makes them superior when it comes to use in agriculture, on country and unpaved roads, at slow speeds and over rough terrain. They are also usually cheaper. Bias ply has been around much longer—radial tires were introduced around 1947—and many trailer manufacturers equip their trailers with bias ply tires. Decades ago, bias ply tires were known to have a higher load-carrying ability, but that’s no longer true as today’s ST steel-belted radial tires have closed that gap.
Why Tire and Wheel Packages Can Mean All Kinds of Savings
Getting trailer tire and wheel packages can you time, save you money and save you frustration. Here’s why. First, with Treadworld Trailer Tire and Wheel Assemblies you can depend on getting quality components in a precision-mounted and balanced package with valve stem. Every RubberMaster Tire is manufactured to strict tolerances from top rubber compounds, then triple-tested for quality, balance and uniformity before being X-rayed to be sure they’re perfect. It is then matched to a tough, durable, virtually indestructible SteelMaster Rim that undergoes its own battery of quality tests before it leaves the factory. The keyword in that sentence is “matched.” You’ve got a wheel and tire package you can depend on for the long haul, literally, without spending time choosing separate components, and without the concern that you might choose mismatched components. That’s a plus for time savings, and also for safety. Plus, you save more time and money by eliminating the need for mounting the tire to the wheel and installing the valve stem. That has already been expertly done and balanced. You’ll also find that it costs you less money than buying the same tire and the same wheel separately—and that’s before you take those components to be mounted and a valve stem added.
Finally, the selection of high speed tire and rim packages for your trailer is impressive, and includes All Steel ST Radials with a Modular Wheel and All Steel ST Radials with an 8-Spoke Wheel.
Though it doesn’t seem like something you should have to consider when selecting the trailer tires you need, age is a factor because tires can begin to degrade even if they’re unused and sitting on a shelf. Typically, each tire manufacturer has a different recommendation on the service life of a particular type of tire, but statistics suggest the average life of a trailer tire is about five years under normal use and maintenance conditions, and that tire replacement should be considered after three years—without regard to treadwear and hours on the road. There’s an easy way to tell how old your trailer tires are—you can find it right on the sidewall. When it comes to the high performance trailer tires you get at Treadworld, you can depend on getting tires that are less than 6 months from their date of manufacture, or in the case of some tires that are less in demand, no more than one year old.
When You Need Trailer Tires, Count on Treadworld
There’s a ton of information in this ultimate guide to choosing trailer tires, but the bottom line is that we here at Treadworld have you covered. We know tires, and for us, customer service is number one! When you’re looking for new trailer tires, or a spare trailer tire, depend on us here at to provide you with the high-performing, long-lasting trailer tires you want, in a wide range of styles and a huge selection of sizes. All our RubberMaster Trailer Tires are manufactured to strict tolerances from top rubber compounds, then triple-tested for quality, balance and uniformity before being X-rayed to be sure they’re perfect. Easy ordering and fast shipping plus your satisfaction is guaranteed with our no-hassle Ultimate Advantage Warranty. And when you’re looking for information, don’t hesitate to contact our tire experts via live chat or email with any questions you may have, and to get the ideal trailer tires —or the perfect ATV tires, UTV tires, lawn and garden tires, and many others—from our extensive selection.
