How to read a trailer tire size
Take ST225/75R15 as the example:
- ST: special trailer. The tire is made for trailer use only.
- 225: section width in millimetres, 8.86 in.
- 75: aspect ratio. The sidewall is 75% of the width, 168.75 mm or 6.64 in.
- R: radial construction. D means diagonal (bias ply) and B means belted bias.
- 15: rim diameter in inches.
After the size you will usually see a load range letter, such as C, D or E. It sets the ply rating and the maximum load and pressure, which are moulded on the sidewall next to it.
The formula
sidewall (mm) = width (mm) × aspect ÷ 100
diameter (in) = rim + 2 × sidewall ÷ 25.4
revs per mile = 63,360 ÷ (π × diameter in inches)Worked example
ST225/75R15: the sidewall is 225 × 0.75 = 168.75 mm, or 6.64 in. The diameter is 15 + 2 × 6.64 = 28.29 in (718 mm). One mile is 63,360 in, and the tire rolls 88.87 in per turn, so it turns about 713 times per mile.
Going from ST205/75R15 to ST225/75R15 on the same 15 in rim adds 1.18 in of height, so the trailer sits about 0.59 in higher at the axle, and 0.79 in of width.
Bias trailer sizes in inch notation
Older and smaller bias trailer tires often use inch codes like 4.80-12, 5.70-8 or 8-14.5, or letter codes like B78-13. The first number in an inch code is the nominal width in inches and the last is the rim diameter. These codes do not state an aspect ratio, so the height cannot be worked out from the code alone. Read the maker's data sheet for those.
What this chart does not cover
This page gives sizes only. The load a tire can carry and the pressure it needs come from the tire itself and from your trailer's placard, not from the size. Replacing a tire with another of the same size does not mean the same load capacity: compare the load range and the maximum load on the sidewall.
Questions
How tall is an ST225/75R15 trailer tire?
28.29 in (718 mm) by the size-code formula: 15 in rim plus two sidewalls of 6.64 in. It is 8.86 in wide and turns about 713 times per mile.
What does ST mean on a trailer tire?
Special trailer. ST tires are made for trailer use only. The load and pressure they are rated for are moulded on the sidewall.
Is ST205/75R14 the same height as ST205/75R15?
No. Same width and sidewall, but the 15 in rim makes it 1 in taller: 27.11 in against 26.11 in.
What load range do I need for my trailer?
The size code does not tell you. Check the tire and loading placard on your trailer and compare with the maximum load moulded on each tire's sidewall.