HOW TO READ A TRAILER TIRE
Much of the information you need to evaluate your tires, and use them safely, will be recorded on their sides in raised letters and numbers.
DOT stands for Department of Transportation; these letters are followed by a string of letters and/or numbers, which are the manufacturer’s codes. The last four digits of this string indicate the week and year a tire was made. So, for example, the digits 1116 indicate that the tire was manufactured in the 11th week, or March, of 2016.
ST means “special trailer,” which is specifically designed for use on trailers. (LT tires are designed for use on light trucks; P indicates the tire was designed for use on passenger vehicles.) These letters will be large and followed by other numbers, which indicate the size of the tire. For example, 225/75R15 means that the tire is 225 millimeters wide, with a sidewall height that is 75 percent of the width. The R indicates a radial tire; a D (diagonal) would mean that the tire is bias ply. The 15 refers to the diameter of the rim, in inches.
Max load indicates the maximum weight a single tire was designed to support. Usually, you’ll see one or two numbers in pounds. For example, a tire might be able to carry 1,480 pounds if used on a single axle trailer or up to 1,280 pounds if used on a tandem axle trailer. Multiply the max load number on each tire by the number of tires on the trailer to determine the maximum weight you can tow. If your loaded rig is heavier than that, you’ll need to upgrade your tires.
The max psi, followed by a number, such as 45, is the maximum inflation pressure (in pounds per square inch) of the tire. You’ll need a good tire gauge to check and monitor the inflation of your tires. Just looking at the tires isn’t good enough. Another handy tool is an air compressor that can be powered by plugging into the cigarette lighter of your tow vehicle.
A number followed by “mph” is the tire’s maximum safe speed. This information isn’t required by the DOT, so not all tires show this. But most trailer tires are rated for highway speeds of 65 mph. Maintaining higher speeds for long distances may cause tires to overheat. Some tires may be rated for maximum speeds designated by letters: L indicates a maximum speed of 75 mph, and M is a maximum speed of 81 mph.