Understanding Trailer Tire Sizes

Trailer tires use three different marking conventions to indicate tire size: numeric, alpha numeric and metric.

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Metric

This is what is found on most Hi-Lo trailers. Most of the newer models use an ST205/75R15C

Metric sizes give the most information about the tire. This includes the addition of alphabetical codes that indicate tire application/type, section width in millimeters, aspect ratio, tire construction, rim diameter (inches) and load range.

ST205/75R15C

ST: Special Trailer
205: Cross section width in millimeters
75: Aspect ratio (75% of cross section width)
R: Construction code for Radial
15: Rim size in inches
C: Load range

Cross section times aspect ratio times 2 (the times 2 is because you have the section width at the top and bottom of the tire) divided by 25.4 (to convert to inches) plus the rim size will give you the overall diameter of the tire.

((205 X .75 X 2) / 25.4) + 15 = This tire is 27.11 inches overall diameter

WARNING: Never use passanger car or light truck tires on a trailer
Special Trailer (ST) tires are built tougher than P and LT tires to withstand more abuse. The ST tire's stronger side wall also helps reduce trailer sway.


See the attached diagram for more details
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Numeric

Numeric identification is the most commonly used convention for indicating the size of small trailer tires. Most numeric identification systems indicate tire Section Width in inches, Rim Diameter in inches and Load Range. Load range is the weight carrying capacity of an individual tire represented by an a letter.

4.80 x 8B

Section Width: 4.80"
Rim Diameter: 8"
Load Range: B


Some larger trailer tires utilize a numeric size that also includes the Overall Diameter. This number is placed at the beginning of the tire's size description.

20.5 x 8.0 x 10C

Overall Diameter: 20.5"
Section Width: 8.0"
Rim Diameter: 10"
Load Range: C


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Alpha Numeric

Alpha Numeric tire size give the Section Width, Aspect Ratio, Rim Diameter and Load Range. Aspect Ratio is the ratio between tire height and Section Width as a percentage. Aspect Ratio is determined by dividing the tire's section height by its section width. The higher the Aspect Ratio, the taller the section height of the tire.

B78 x 13C

Section Width: B
Aspect Ratio: 78%
Rim Diameter: 13
Load Range: C

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Attached Images

tire_size_dimensions.jpg (11.9 KB, 48 views)

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2001 Hi-Lo 22TL
2010 F150