Towing a trailer is an undertaking that must be taken seriously for safety
reasons. The load being towed and the size of the trailer must be able to handle
the job. The trailer tires should be sufficient to handle the load being
Measuring a trailer tire involves sets of numbers that stand for the
measurements and specifications of the tire. Understanding these markings will
help in deciding what trailer tire is needed.
Begin by laying the trailer tire on its side on the ground. When measured
from the ground to the edge of the tire, that equals the section width. In this
example we will use a common trailer tire with the markings ST225/75R15. This is
a metric value that is found on the sidewall of the tire. This measurement from
the ground up should correspond to the first value, in this case 225, in
millimeters. To convert this value to numeric, the formula is M x .0394. To
convert our example, 225 millimeters x .0394 = 8.865 inches.
Measure aspect ratio from the top of the trailer rim to the top of the
tire. In our example, ST225/75R15, this is the second number or 75. This stands
for the number, as a percentage, to subtract from the section width, which
results in the value of the trailer tire's section height. When using our
example, the formula would be .75 x 225=169. Our section height is now 169
millimeters. The conversion formula to inches is again, M x .0394. Take 169 x
.0394 = 6.658 inches.
Internal construction is contained in a metric label only. This is
identified by one letter, being R or B or D. R is radial construction, so the
tire's body plies extend from the center. D is for diagonal or bias ply
construction, so the tire's body plies pattern in a crisscross. B is for belted
construction, so the tire's body plies pattern in a crisscross and is
strengthened with belts. In our example, the R is for radial construction.
Following this letter is a number representing the diameter of the trailer
wheel. The third value is the wheel diameter stamped on a trailer tire. In our
example, ST225/75R15, this value is 15. Even though this is called a metric
measurement value, the last value in these labels are inches. In our example, 15
stands for 15 inches. This value is obtained by measuring the space in the
middle of the tire or the trailer rim.
Using ST225/75R15 again as an example, it shows two letters, ST, which
signifies the application type. In this example, ST is the value for special
trailer. There are other codes that represent other application types. P
=passenger vehicle, T= temporary spares, LT=lightweight trucks and C=commercial
In summary, there are three areas to measure to decide the correct trailer
tire size required. The width of the trailer tire, the diameter of the rim and
the height from the rim top. The letters correspond to the appropriate load use
or the materials used in making the tire. The best way to ensure road safety is
to have these values as accurate as possible. Use these values to determine that
the load being hauled by the trailer have sufficient capabilities; and are
appropriate to the load being hauled.