Trailer Tire Load Rating: What It Means and Why It Matters
Towing anything — a boat, a camper, a flatbed of equipment — puts unique demands on tires that most passenger car owners never have to think about. Trailer tires carry heavy, static loads without the benefit of engine braking or steering input. That's why load rating is the single most important number on a trailer tire, and why getting it wrong creates real safety risk.
What a Tire Load Rating Actually Tells You
A tire's load rating (also called load capacity or load index) tells you the maximum weight a single tire can safely carry when inflated to its rated pressure. It's not a suggestion — it's an engineering limit.
On trailer tires, this number appears in two places:
- The load index — a two- or three-digit number on the sidewall that corresponds to a standardized weight table. A load index of 100, for example, corresponds to 1,764 lbs per tire.
- The load range letter — typically B, C, D, or E on trailer tires, indicating the number of plies and the tire's maximum inflation pressure. Higher letters mean stiffer construction and higher load capacity.
These two figures work together. You can't evaluate one without the other.
ST vs. LT vs. P: Why Trailer Tires Are Different
Most trailer tires are marked ST (Special Trailer). These are built differently than LT (Light Truck) or P (Passenger) tires, and that difference matters for load rating comparisons.
| Tire Type | Designed For | Sidewall Strength | Typical Load Range |
|---|---|---|---|
| ST | Trailers only | Very stiff | C, D, or E |
| LT | Trucks/SUVs towing | Reinforced | C, D, or E |
| P | Passenger vehicles | Standard | Standard load / XL |
ST tires are engineered for high load, low speed, and minimal lateral stress. Their stiff sidewalls resist the swaying forces trailers generate. Using passenger tires on a trailer axle — even ones with similar load ratings — puts the tire in a use case it wasn't built for.
How to Calculate the Load Rating You Need 🔢
Here's the basic math:
- Find your trailer's GVWR (Gross Vehicle Weight Rating) — this is listed on the trailer's VIN plate or owner documentation.
- Divide by the number of tires on the trailer.
- The result is the minimum load rating per tire you need.
Example: A trailer with a 7,000 lb GVWR and four tires needs tires rated for at least 1,750 lbs each — which corresponds roughly to a load index of 100.
Most trailer owners are advised to leave a margin above that minimum, not just meet it exactly. Running a tire at its absolute rated maximum continuously, especially at highway speeds or in heat, shortens its life and increases failure risk.
The Role of Inflation Pressure
A tire's load capacity is only valid at its maximum rated inflation pressure. Underinflate the tire, and the load rating drops — sometimes significantly. This is one of the most common causes of trailer tire blowouts.
ST tires are typically rated at 50, 65, or 80 PSI depending on load range. That's considerably higher than a typical passenger car tire. Many drivers underinflate trailer tires because they're applying passenger-car habits to a completely different type of tire.
Load range D and E tires, in particular, need to be checked cold before every significant trip. Trailer tires don't have TPMS sensors in most cases, so there's no dashboard warning if pressure drops.
Speed Rating and Its Effect on Load Capacity ⚠️
Most ST tires carry a maximum speed rating of 65 mph. Some are rated for only 60 mph. Exceeding that speed reduces the effective load capacity — typically by 10% for sustained speeds above 65 mph, and more beyond that.
This matters because highway driving at 70–75 mph is common. If you're towing at those speeds, you're operating at reduced rated capacity even with properly inflated tires — which is another argument for leaving headroom above the minimum load rating.
Load Range and Trailer Type
Different trailer categories tend to use different load ranges as a starting point:
- Utility and small cargo trailers — often Load Range C (6-ply rated)
- Boat trailers and mid-size cargo trailers — often Load Range D (8-ply rated)
- Heavy-duty flatbeds, large RVs, fifth wheels — often Load Range E (10-ply rated)
These are general patterns, not rules. A small trailer carrying dense equipment might need a higher load range than a large but lightly loaded camper.
What Changes the Calculation
The right load rating for any trailer depends on variables that aren't universal:
- Actual loaded weight, not just GVWR — cargo distribution matters, especially on tandem-axle trailers
- Number of axles — single-axle trailers put 100% of the load on two tires; tandem-axle trailers distribute it across four
- Tire size — the same load range in different sizes carries different maximum weights
- Ambient temperature — heat increases pressure and accelerates wear; cold decreases effective pressure
- Road conditions and trip frequency — weekend use versus daily commercial hauling changes what "adequate" really means
The right tire for a weekend boat trailer driven 50 miles on rural roads isn't necessarily the right tire for a commercial flatbed making weekly interstate runs — even if the GVWR is the same.
The Gap That Matters
Load rating is stamped right on the sidewall, but what that number means in practice depends entirely on what you're hauling, how far, how often, and at what speeds. Two trailers with identical GVWRs can require meaningfully different tires based on how they're loaded and used.
Your trailer's documentation, the VIN plate, and the tire manufacturer's load tables are the starting points — but your actual load, your route, and your towing conditions are what determine whether a given tire is working within its limits or against them.
