Measuring a Trailer Tire

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.

• Metric:
  • ST: Stands for "Special Trailer," meant to be used on boat or utility trailers
  • P: Stands for "P-metric," meant to be used on passenger vehicles, such as cars, vans, light-duty trucks and SUVs
  • T: Stands for "Temporary Spare," meant to be used until the original tire can be replaced
  • C: Stands for "Commercial" (used only in Euro-metric sized tires) meant to be used on vans or delivery trucks capable of hauling heavy loads
  • LT: Can have different meanings depending on where it falls in the tire size:
    • Beginning of a tire size: Stands for "Light Truck-metric," meant to be used for vehicles capable of hauling heavy cargo and pulling large trailers
    • End of a tire size: Stands for "Numeric" (carrying heavy cargo and pulling trailers), "Wide Base"(using 16-1/2 inch diameter rims) or "Flotation" (designed for driving on loose dirt or sandy surfaces)
• Numeric: Does not list the application type.
• Alpha Numeric: Does not list the application type.

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

2. Section Width

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.

• Metric: A three digit number follows the letters that identify the service type. This number designates the width of the tire in millimeters. For example, if a tire's size was ST175/80-R13 LRD, the 175 identifies the section width as 175 millimeters.
• Numeric: The first set of numbers identifies the section width in inches. For example, if a tire size is 4.80 X 12B, then you know the section width is 4.80 inches.
• Alpha Numeric: Does not list the section width.

3. Aspect Ratio

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

• Metric: The two digit number that follows the section width, ST175/80-R13 LRD, is the aspect ratio. To find the section height, you take 80% of 175 which is 140 millimeters, so the section height is 140 millimeters. To convert to inches, take 140 mm X .0394 (conversion unit for millimeters to inches); the section height is 5.516 inches.
• Numeric: Does not list the aspect ratio
• Alpha Numeric: The two digit number that follows the first letter, B80 X 15D is the aspect ratio. Use the same method above to find the 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.

• R: Stands for radial construction, meaning the tire's body plies branch out from the imaginary center of the wheel.
• D: Stands for diagonal or bias ply construction, meaning the tire's body plies crisscross.
• B: Stands for belted construction, meaning the tire's body plies crisscross as bias ply does and then are reinforced with belts.

• Metric: The letter that follows the aspect ratio, ST175/80-R13 LRD, signifies the tire's internal construction.
• Numeric: Does not list the internal construction.
• Alpha Numeric: Does not list the internal construction.

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.

• Metric: In the example ST175/80-R13 LRD, the 13 indicates the wheel diameter in inches.
• Numeric: In the example 4.80 X 12B, the 12 indicates the rim wheel diameter in inches.
• Alpha Numeric: In the example, B80 X 15D, the 15 indicates the wheel diameter in inches.

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.

• Metric: In the example, ST175/80-R13 LRD, the LRD indicates D as the load range.
• Numeric: In the example, 4.80 X 12B, the B indicates the load range.
• Alpha Numeric: In the example, B80 X 15D, the D indicates the load range.

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.

• Load Range B: 4 Ply Rating, 35 psi
• Load Range C: 6 Ply Rating, 50 psi
• Load Range D: 8 Ply Rating, 65 psi
• Load Range E: 10 Ply Rating, 80 psi

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.

• Load Index: Number that correlates to the load carrying capacity of a tire
• Speed Rating: Indicates maximum speed for a tire, assuming the load is at or below what the tire is rated for

• Metric: The last number and letter combo in P-metric size tires indicate the service description.
• Numeric: Does not list the service description.
• Alpha Numeric: Does not list the service description.