Measuring a Trailer Tire

trailer tire size

Each trailer tire has a size branded to its sidewall. This size lists valuable information about your tire. The seven pieces of information that a tire size may contain are the following:

  1. Application Type
  2. Section Width
  3. Aspect Ratio
  4. Internal Construction
  5. Wheel Diameter
  6. Load Range
  7. Service Description
    • Includes Load Index and Speed Rating

Writing Tire Sizes

There are three conventions for writing tire sizes. Each method includes some or all of the information mentioned above.

  1. Metric: This way of writing tire sizes contains the most information about the tire; an example is ST175/80-R13 LRD.
    • Includes the application type, section width, aspect ratio, tire construction, rim diameter and load range
  2. Numeric: This is the most common way of writing small trailer tire sizes; an example is 4.80 X 12B.
    • Includes the section width, rim diameter and load range
  3. Alpha Numeric: This method is not used on any of the tires we sell; an example is B80 X 15D.
    • Includes the aspect ratio, rim diameter and load range

1. Application Type

The application type indicates what the tire is intended to be used for.

Note: When replacing trailer tires, we recommend using Special Trailer (ST) tires.

2. Section Width

section width of trailer tire

The section width refers to the distance from the widest point of a tire's outer sidewall to the widest point of its inner sidewall; this distance is measured once the tire is mounted on the wheel it is going to be used with.

3. Aspect Ratio

section height of trailer tire

Sidewall aspect ratio (or section height) is the ratio of the tire height and width that will tell you the tire's section height.

4. Internal Construction

Internal construction refers to how a tire is made. The letters below indicate the possible internal constructions a tire can have.

rim diameter

Note: For everyday use, tires with different internal construction cannot be used together.

5. Wheel Diameter

Wheel (or rim) diameter refers to the diameter of the wheel the tire is mounted on.

6. Load Range

The load range indicates the horizontal and sidewall strength of a tire. It is represented by a letter within each tire size. Each load range letter correlates to a specific number of tire plies and a load pressure in psi units. Sometimes the load range is left off the tire size and is branded on the tire elsewhere.

The information below tells you the ply rating and load pressure (in psi units) for each load range for Special Trailer (ST) and Light-Truck (LT) service tires.

Note: The weight capacity at a certain unit of psi for each tire is branded on each tire. It may be listed under the tire size or somewhere else on the tire.

7. Service Description

The service description includes the load index and speed rating. Only Passenger (P-metric) tire sizes include the service description.

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