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Updated November 2026
Trailer tires fail more often from overloading and underinflation than from anything else. Getting the load capacity right is simple once you know three numbers: your trailer’s GVWR, the number of tires, and the load capacity printed on each tire’s sidewall. This guide shows you how to calculate what you need, how load ranges work on ST trailer tires, and how to keep your tires inside their limits.
Key takeaways
- Total tire capacity must exceed your trailer’s GVWR. Divide GVWR by the number of tires, then add a safety margin.
- Load capacity depends on inflation pressure. ST tires reach their rated load only at the maximum cold pressure on the sidewall.
- Load ranges C, D, E, F and G indicate strength and maximum pressure.
- Many ST tires are speed-rated for 65 mph unless the sidewall shows a higher speed symbol.
How to calculate the load capacity you need
- Find your trailer’s GVWR (Gross Vehicle Weight Rating) on the VIN or certification label, usually at the front of the trailer tongue or frame. This is the trailer’s maximum loaded weight.
- Subtract tongue weight if you like, but most owners don’t. Using the full GVWR builds in a safety margin, since some weight sits on the hitch.
- Divide by the number of tires on the ground. Example: a 7,000 lb GVWR tandem-axle trailer has four tires, so each must carry at least 1,750 lbs.
- Add a safety margin. Many owners and tire shops aim for roughly 10–20% more capacity than the minimum to allow for uneven loading, heat and hard roads. In the example, that means tires rated around 1,950–2,100 lbs each.
- Weigh your loaded trailer at a public scale (such as a truck stop) to check your real weight, and check each side if you can. Uneven loading can overload tires on one side.
ST trailer tire load ranges
| Load range | Ply rating | Typical max cold pressure (ST) | Common use |
|---|---|---|---|
| B | 4-ply | 35 psi | Small utility trailers |
| C | 6-ply | 50 psi | Utility, small boat and camping trailers |
| D | 8-ply | 65 psi | Mid-size travel trailers, car haulers |
| E | 10-ply | 80 psi | Larger travel trailers, toy haulers, equipment trailers |
| F | 12-ply | 95 psi | Heavy fifth-wheels and equipment trailers |
| G | 14-ply | 110 psi | Very heavy fifth-wheels and commercial use |
“Ply rating” describes strength, not the actual number of layers. Always check the exact pressure and load printed on your tire’s sidewall, as they vary by size and brand.
Typical load capacities of common ST sizes
| Size | Load range | Approx. max load per tire |
|---|---|---|
| ST205/75R15 | C | about 1,820 lbs |
| ST205/75R15 | D | about 2,150 lbs |
| ST225/75R15 | D | about 2,540 lbs |
| ST225/75R15 | E | about 2,830 lbs |
| ST235/80R16 | E | about 3,420 lbs |
These are typical figures at maximum cold pressure. Your tire’s sidewall is the final word.
Why inflation decides load capacity
A tire’s rated load is only available at its rated pressure. Most trailer and tire makers recommend running ST tires at the maximum cold pressure shown on the sidewall. Running 10 psi low can cut capacity significantly and cause heat build-up, which leads to tread separation and blowouts. Check pressure cold, before every trip, with a reliable gauge, and consider a trailer TPMS for long trips.
Speed matters too
Traditionally ST tires were rated for 65 mph, and many still are. Some newer ST tires carry higher speed symbols (for example L = 75 mph or higher). Check the sidewall. If your tires are rated 65 mph, driving faster increases heat and the risk of failure, so stay within the rating.
Can you use LT or passenger tires on a trailer?
LT tires are used on some trailers, especially off-road and overland trailers. Passenger (P-metric) tires are not recommended. If a non-ST tire is used on a trailer, tire makers typically require derating its load capacity, so it must have even more capacity than your calculation. Check the tire maker’s guidance.
When to replace trailer tires
- When tread reaches the wear bars, or shows cuts, bulges or cracking.
- When they reach the age limit recommended by the tire or trailer maker. Trailer tires often age out before they wear out, because they sit unused in the sun. Many owners replace them every 3–5 years.
- After any tire failure, inspect the others on the same axle.
Trailer tire load checklist before every trip
- Check pressure cold, at the sidewall maximum for ST tires.
- Look for cracks, bulges and uneven wear.
- Check DOT age.
- Load heavy items over the axles and balance side to side.
- Stay within the tire’s speed rating.
- Check the spare.
Shopping for new tires? See trailer tire brands for off-road use and our trailer tire guides. For general pressure tips, read our tire PSI guide.
Frequently asked questions
How do I know what load range trailer tires I need?
Divide your trailer’s GVWR by the number of tires, add a 10–20% margin, and choose a size and load range whose sidewall capacity meets that number.
Is it OK to put higher load range tires on a trailer?
Yes, as long as your wheels are rated for the higher pressure and load. Check the wheel’s rating, often stamped inside the rim.
Should trailer tires be inflated to max psi?
Most trailer and tire makers recommend inflating ST tires to the maximum cold pressure on the sidewall to get their full load capacity.
How long do trailer tires last?
They usually age out before wearing out. Many owners replace them every 3–5 years, depending on use, storage and the maker’s guidance.