Trailer Tire Size Calculator
Compare ST tire sizes for trailers. Use the calculator below. Results update as you type.
How the Trailer Tire Size Calculator Works
This calculator works by decoding Special Trailer (ST) sizes into diameter, width and revs per mile, and comparing them with other trailer sizes. ST tires are built only for trailer use.
- Enter the size, for example ST225/75R15.
- Read the diameter: 28.29 in (719 mm).
- Read the width: 8.86 in (225 mm).
- Read the revs per mile: 713.
The Formula
The formula is the standard diameter formula applied to ST sizes.
D (in) = Rim + 2 × (Width × Aspect Ratio ÷ 100) ÷ 25.4 Revs per Mile = 20,168 ÷ D (in)Worked example: ST235/80R16 has a 188 mm (7.40 in) sidewall and a 30.80 in diameter, so it turns 655 times per mile.
Conversion Table
The conversion table lists 8 ST sizes on 13 in to 16 in rims. Diameters run from 24.02 in (ST175/80R13) to 31.73 in (ST235/85R16).
ST tires on the same trailer axle must match. A height gap between tandem axles shifts load onto the taller tires.
| ST Size | Diameter (in) | Width (in) | Revs / Mile |
|---|---|---|---|
| ST175/80R13 | 24.02 | 6.89 | 840 |
| ST205/75R14 | 26.11 | 8.07 | 773 |
| ST215/75R14 | 26.70 | 8.46 | 755 |
| ST205/75R15 | 27.11 | 8.07 | 744 |
| ST225/75R15 | 28.29 | 8.86 | 713 |
| ST235/80R16 | 30.80 | 9.25 | 655 |
| ST235/85R16 | 31.73 | 9.25 | 636 |
| ST185/80R13 | 24.65 | 7.28 | 818 |
What Is the Trailer Tire Size Calculator?
The Trailer Tire Size Calculator is a tool that processes ST trailer tire sizes and compares their dimensions. ST tires use stiffer sidewalls to resist trailer sway and carry heavy loads at moderate speed.
Many ST tires carry a lower speed limit than passenger tires; older ST tires default to 65 mph (105 km/h) unless marked otherwise. Never fit ST tires to a car or truck.
Common Uses
This calculator has 4 uses: trailer replacements, tandem matching, boat trailer checks and camper checks.
- Replace trailer tires: Decode ST sizes.
- Match tandem axles: Keep all tires the same height.
- Check boat trailers: Compare small-rim ST sizes.
- Check campers: Size tires to the trailer's weight.