When it comes to buying trailer tires, the numbers you’re probably most concerned about are size and price, but there are a host of digits—and letters—imprinted on the sidewall of your tires, and knowing what they mean helps you better understand how your tires work, and how you can choose the safest trailer tires for your needs.

Tire class and size:

ST215/75R14

The first thing you want to see is ‘ST,’ which stands for Special Trailer tire. With their heavy-duty construction, trailer tires are built to tow heavy loads, withstand excessive heat, and reduce sway. Trailers can only use ST tires, and ST tires can only be used on trailers.

After ‘ST,’ you’ll see the numbers used to indicate your trailer tire’s size: 215, width in millimetres; 75, aspect ratio or ratio of height to width; R for radial construction; and 14 for rim diameter.

Load rating

Load rating or load index tells you how much weight a tire can safely carry at its maximum air pressure. At its maximum air pressure of 50 PSI (cold), a load range ‘C’ tire might have a load rating of 1760 pounds.

Load range

Load range tells you the type of load a tire is designed to support at a specific inflation pressure. Trailer tires typically have C, D, or E load ranges. A load range ‘C’ tire, for example, is at its peak load capacity—possibly 1600 pounds—when it’s inflated to its maximum pressure of 50 PSI. A load range ‘C’ tire at 25 PSI might be able to support a load of 990 pounds, while at 40 PSI, that capacity could be 1300 pounds.

Max press.

This tells you the maximum pressure (when your tires are cold) needed for your tires to carry its maximum load, in PSI.

DOT

This stands for the U.S. Department of Transportation.’ There will be about 10-12 numbers following ‘DOT.’ The first six to eight numbers indicate the manufacturer’s code, where the tire was manufactured and the tire size.

Date code

To find out when your tire was made, look for the serial number that begins with ‘DOT.’ The last four numbers indicate the week and year your tire was made, respectively. A date code of 2615 means the tire was made in the 26th week of 2015. Some trailer tire manufacturers suggest three to five years is the average life expectancy of a trailer tire, regardless of mileage.

Tire type

Below the tire size, you’ll see either ‘radial’ or ‘bias.’ Radial tires (or ‘radial-ply tires’) are constructed with polyester and/or nylon plies that run across the tire perpendicularly, and sometimes include steel belts that run under the tread. Bias-ply tires Bias-ply cords layer in a criss-cross pattern from sidewall to sidewall, and they are also sometimes reinforced with a steel belt.

Puncture Resistance of Sidewalls

There is no specific sidewall marking that identifies the puncture resistance of a tire; however, a rough link can be made about durability based on the ply rating, tire construction and the application that the tire is used in. For example, a tire designed for highway use will not tolerate rough, rocky driving. An all-terrain tire which has been designed to handle off-road conditions would be a better choice.

A tire with LT 265/70r17 E markings on the sidewall tells us it’s designated for use on a “light truck” and the letter “E” indicates it is a 10 ply equivalent tire, keeping in mind that “plies” is an older term which is now more commonly referred to as “load range”.

Since newer materials perform better, a tire doesn’t need as many layers and makes for a lighter tire that runs cooler and performs better.

These load ranges show what the equivalent ply is:

Load range “E” = 10 ply equivalent

Load range “D” = 8 ply equivalent

Load range “C” = 6 ply equivalent

Older bias ply tires have sidewall markings that indicate how many cotton plies are in the tire, whereas most current/modern tires contain newer polyester, nylon-type materials and usually have only 2-3 carcass plies.

Also, the higher the load range, the more weight carrying capacity of the tire. To carry that extra weight, more air pressure is needed, and to hold that extra air pressure, a more robust carcass is used, which usually adds more durability.