How to Read Trailer Tire Sizes

How to Read Trailer Tire Sizes

When shopping for new trailer tires, you may have noticed specific information inscribed on the sidewall. Learning how to read trailer tire sizes can be confusing, especially if you haven’t owned a trailer for very long.

Do the imprinted letters reveal what type of service your vehicle can perform? Do the various numbers have specific meanings? We’ll address all these questions in the following guide so that you can know how to read trailer tire sizes accurately.

Short Answer

The imprinted inscription with bold letters and adjacent numbers can tell you not only your tire class but also the wheel’s width, aspect ratio, internal construction, and rim diameter code.

Identifying the Trailer Tire Class and Size

The tire class is the two-letter classification of what kind of service the vehicle’s wheel can perform, and the size gives you the details of the wheel’s height, width, and diameter. This information can help you understand the specifics of your exact wheel when you’re searching for a new one. 

how to read trailer tire size


If you have a vehicle with a heavy load, you want to make sure your tire starts with the letters “ST,” which stands for “Special Trailer” tire. As you could probably tell by its name, this accessory will only work on trailers. ST wheels have stronger sidewalls, which allow them to carry heavier loads. In addition, they can withstand excessive heat and are sturdy enough to reduce sway.


There are also “LT” tires, which stand for “Light Truck.” LT wheels were designed for vehicles to carry large loads and towing vehicles — you shouldn’t install them on the frame of a trailer. Unless you account for the load rating, we recommend you avoid using LT tires on a trailer — stick to ST ones.


how to tell trailer tire sizes

Three digits follow the class and the letters of the wheel — these three numbers represent the normal width of the tire. The numbers are the section width or cross-section width of the wheel in millimeters. When looking at a bird’s eye view, this is the tire’s width when the wheel is upright.

Aspect Ratio

Following the two letters and the three-digit number is a slash. After the slash, you’ll see a two-digit number, which represents the aspect ratio of the wheel. The aspect ratio is the ratio of the height and width of the wheel. You can also think of it as the height of the sidewall expressed as a percentage of the cross-section width. 

Internal Construction

how to tell trailer tire size

Following the two-digit aspect ratio, you’ll see one letter — this character represents the internal construction. There are two different kinds of constructions — bias-ply or radial. 

If the tire has the letter “D,” the wheel has a bias-ply or diagonal construction. If it has the letter “R,” the wheel has a radial construction.

Rim Diameter Code

The final two-digit number present is identification size, which indicates the diameter of the wheel the tire is mounted on. This number is measured in inches. For example, if you see the number 16, your wheel will fit a rim with a 16-inch diameter.

Why You Should Know How to Read Trailer Tire Sizes

Trailer tires can tow large objects for many miles at a time. If you install ones of the wrong class and size, you could experience the following hazards:

how to find out trailer tire sizes

Poor Turning Radius & Fuel Efficiency

Incorrectly sized wheels can negatively affect your vehicle’s driving performance. For example, wheels that are too big won’t provide you with the ideal turning radius. On the other hand, ones that are too small will lower your gas mileage.

Accidental Tire Blowout  

The classification letter on regular passenger vehicles has the letter “P.” If you try to use one of these passenger vehicle tires on a trailer, the pressure of the trailer will cause significant tread wear. The wheel’s weight capacity was not designed to bear such a heavy load, so a blowout could easily occur.

Safety Concerns

find out trailer tire size

If you use a passenger tire with the wrong size dimensions, you are running the risk of a wheel blowing out on a major highway. The wrong sized one could cause serious harm to your trailer, yourself, or other drivers on the road. 

The Bottom Line

When looking to buy a new trailer tire, it’s important to know how to read trailer tire sizes. The markings indicate whether the wheel is meant for larger vehicles and inform you of the wheel’s width, aspect ratio, construction, and diameter code.

You’ll need the exact classification and size numbers to make your next wheel the perfect match for the other ones. If one of the wheels can’t carry the stress of the trailer, your vehicle could make a trip to the mechanic or become a safety hazard.

Did you find this blog helpful? Then feel free to checkout other recommended readings:

How To Measure Tire Rim Size
How To Convert Tire Size To Inches
How Do You Measure Tire Size
How To Find Out Tire Size
How To Find Tire Diameter