Whether they undergird a large fifth-wheel or a small teardrop camper, all RV and trailer tires share some common DNA.
They are the humble rubber servants of your RV or utility trailer. Treat ‘em well, and they might outlive the family dog. Treat ‘em rough, and you’ll find yourself on a first-name basis with your local tow truck driver.
You need to choose your RV tires carefully. They are the life-or-death connection between your rig and the road. So don’t skimp.
And you don’t have to rifle through your Owners Manual to learn how. Especially if you’ve already “misplaced” that Manual … now where was it??
Before we dig in, please be aware that this article series is for towables (travel trailers and fifth wheels) only! Not targeted towards motorhomes, Sprinter vans, etc. It’s still useful if you own a motorhome, but be aware there are differences.
Some Great Resources
Normally, readers, I post resources at the end of an article. Don’t want to interrupt the flow of information and all that.
But these resources are so good, I just had to tell you upfront.
- RVSEF: RV Safety and Education Foundation. RVSEF is a 501(c) non-profit organization dedicated to consumer education and safety. They are endorsed by the RVDA and RVIA and serve up a platter of free safety knowledge. Check out their RV tire safety page and manufacturer tire inflation tables.
- RVTireSafety.net is a blog run by Roger Marble, retired Tire Design and Quality engineer of 40 years. This man has forgotten more about tires than I’ll ever learn. If you have a technical question about RV tire design and selection, this man has probably answered it.
- TireAmerica. This is a great store to purchase tires online. They sell both vehicle and trailer tires. Click here to learn about their price-match guarantee.
BONUS: If you want to know how tires are made, check out this video: CAR TYRES | How It’s Made. The narrator has an English accent, so you won’t get bored listening.
How to Read a Tire Label Placard
Let’s begin at the simplest place: Your existing RV tires.
Take a knee and look at the sidewalls of your tires. You’ll see all sorts of numbers and hieroglyphics. You’re looking for a big, bold alphanumeric sequence like this:
Good work!
Now, you should confirm whether your tires match the factory original sizes.
Search your trailer tongue or left front corner sidewall. You’ll find a tire placard, which is a sticker listing the sizes of all tires, all axles. Your RV manufacturer is required by law to post this sticker.
If your RV is old and gray, then the tire placard might be faded. In that case, rifle through your Owners Manual or contact your RV manufacturer.
Each field in the tire placard should have a tire size label.
ST225/75-R15 LRD M
If we break that down into functions, it looks like this:
[Type][Width]/[Aspect Ratio]-[Ply][Rim Size]-[Load Range] [Speed Rating]
FYI: Not all tire or tire labels will include the speed rating in this sequence, by the way.
Let’s get out your Ovaltine decoder and figure out what this means.
TYPE: ST = “Special Tire”
ST stands for “Special Tire,” a designation given to trailer tires in the Federal Motor Vehicle Safety Standard (FMVSS).
ST tires are designed specifically for trailer use. They have stiffer, heavier side walls than “P” passenger car tires and “LT” truck and SUV tires.
Here’s what Tim Fry, senior development engineer with Goodyear Tire & Rubber Company, says:
“The major difference is reflected in the polyester cords used in ST tires. These cords are bigger than they would be for a comparable P or LT tire. Typically, the steel wire also has a larger diameter or greater tensile strength to meet the additional load requirements. Because of the heavier construction for an equal volume of air space, an ST tire is designated to carry more load than a P or LT tire.”
Tim Fry, Goodyear Tire & Rubber Company
ST tires are also tested to different standards. Passenger vehicle (P) tires may have ratings for tread life, wet traction, seasonality, etc. ST tires usually don’t. They don’t lead; they follow.
SECTION WIDTH: 225 = 225 Millimeters
This is an easy one. It’s the width of the tire, measured in millimeters.
One inch equals 25 millimeters. And don’t start wavin’ that American flag at me! Here is a map of the only countries not using the metric system!
So get with the world, man. The metric system is the shizzle.
ASPECT RATIO: 75 = 75%
This is a little more complicated. The aspect ratio is expressed as a percentage of the section width. So the height of the tire wall (not the diameter), in this case, is equal to 75 percent of the width.
If you’re starting to hyperventilate, just grab a brown bag and sit down. It’s not really important. Or check out this drawing:
Look, the bigger the number, the bigger the tire, alright? You don’t need to whip out a calculator to go tire shopping. Just match numbers. It’s like playing Go Fish with your 4-year-old.
Just understand that, in a roundabout way, the bigger the first number, the wider the tire; the bigger the second number, the taller the tire.
PLY CONSTRUCTION: R = Radial Ply Tires
There are two basic methods of tire construction: radial-ply and bias-ply. Unless you’re a tire geek, you don’t need to understand the manufacturing differences.
- Almost all passenger vehicles and RVs use radial-ply tires.
- Motorcycles, off-road wagons and utility trailers might use bias-ply tires.
If you’re a digital arsonist and enjoy igniting online flame wars, go to an RV forum and type in “radial vs bias ply tires.” Everybody and his mother will have an opinion. Tell ‘em they’re wrong, get some popcorn and watch the fireworks.
So let’s keep it simple. When in doubt, go with radial-ply tires. For RV manufacturers, this is now required by law.
RIM DIAMETER: 15 = 15 inch Rim Size
Now we’re back in familiar territory. This is how lay people normally describe tires “14-inch tires, 15-inch tires, etc.”
This is the measurement of the diameter of the rim the tire is mounted on.
Many tires actually fit a range of rim widths. A 205/75-R15 tire, for example, can often fit on either a 15×5.5 or a 15×5 rim. Your rim width will have a slight impact on the total height and width of the mounted tire and wheel assembly.
Note: This does NOT apply to rim heights! Don’t mount a 16-inch tire on a 16.5-inch rim! Yer gonna die.
Many RV tires come with polished aluminum rims. They weigh less and look better than steel rims.
Steel rims can be stronger and more rigid, though. Even though steel rims are usually relegated to cheap spare tires and utility trailers, some hardcore off-roaders may use steel rims for the added rigidity.
A good sign of quality in a steel rim is the finish. Chrome plating is the best. Powder-coating is (normally) pretty good. Cheap paint won’t last past the dealership’s lot. When it comes to finishing, you really do get what you pay for.
LOAD RANGE: LRD = Load Range D
Load range just indicates strength at the stated inflation pressure. D is stronger than C, E is stronger than D, etc.
Now, I’ve tried to stay away from complicated charts and graphs, but you should know one thing, because we’ll talk about it a lot later:
Load rating varies with pressure!
If a tire is rated for, say, 2140 lbs, that is only at a certain inflation pressure! Less pressure = less strength! Flat tires = weak tires!
Example: You’re running LRD 15-inch tires rated for 2140 lbs each at 65 psi. But you were 60 minutes getting out the door, so you didn’t air up your tires before the Big Road Trip.
So you’re cruising along at 35 psi rather than 65 psi. Your tires aren’t strong enough to carry 2,140 lbs! They’re gasping for air! Maybe they’re only rated for 1,800 lbs now.
Twenty minutes later – pop! Now your tires look like they went through a cheese grater. Don’t be that guy. In fact, we’ll dive into this common problem in great depth in Part 2 of this series.
To whet your appetite, here’s a critical resource: A tire load/inflation chart from Goodyear, manufacturer of USA trailer and RV tires.
As you’ll see, tire pressure, speed and load rating are all intertwined. That means you really need to know how much your camper weighs – even before you buy! Here’s a resource I wrote about average camper weights I recommend.
Air up your tires before every trip! And check them every day.
SPEED RATING: M = 81 mph
There is A LOT of fine print when it comes to tire speeds. For now, here are the chart values.
No speed rating? Assume 65 mph for all ST tires.
Here are some other common speed ratings:
- J: 62 mph
- L: 75 mph
- M: 81 mph
- N: 87 mph
How to Read a Tire Placard
See? It was easy! If you can count to 100, play Go Fish and recite your ABCs, you can read a tire placard label.
Unfortunately, readers, you can’t stop here. Because your RV tires might be overloaded –
(And it might not even be your fault!)
You’ll learn about proper tire pressure, how fast you can drive, and why you need to weigh your RV!
Check out:
… for critical information about how to prevent a tire blowout.
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