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:
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: 225, width in millimetres; 75, aspect ratio or ratio of height to width; R for radial construction; and 15 for rim diameter.
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 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.
This tells you the maximum pressure (when your tires are cold) needed for your tires to carry its maximum load, in PSI.
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.
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.
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.