ST225/75R15 Load Range E is a trailer tire size rated to carry heavy loads at highway speeds

The code ST225/75R15 Load Range E describes a specific trailer tire built to handle weight. Breaking it down: ST means it is a special trailer tire (not for cars or trucks); 225 is the width in millimeters; 75 is the sidewall height as a percentage of width; R means radial construction; 15 is the wheel diameter in inches; and Load Range E means it can carry 3,520 pounds per tire at maximum pressure (80 psi). A trailer with four of these tires can carry up to 14,080 pounds of cargo weight, not counting the trailer frame itself.

This tire size is common on enclosed cargo trailers, utility trailers, and boat trailers that regularly move heavy loads. The Load Range E designation is the highest rating for this tire size, so it is built with thicker sidewalls and more internal reinforcement than lower load range versions of the same tire. If your trailer is rated for this tire, using anything less than Load Range E leaves you underprotected and risks blowouts under load.

Key Takeaways

  • Load Range E means each tire can safely carry 3,520 pounds at 80 psi, so four tires support 14,080 pounds of cargo weight.
  • The ST prefix identifies this as a special trailer tire, not a passenger or light truck tire, and it is built differently to handle the stresses of towing.
  • You must inflate these tires to the full 80 psi when the trailer is loaded; under-inflation causes sidewall flex, heat buildup, and blowouts.
  • Load Range E is the maximum rating for this size, so substituting a lower load range tire creates a safety gap between what your trailer needs and what the tire can deliver.
  • Trailer tire pressure and load capacity do not change with temperature the way car tires do, but you should still check pressure before every trip.

How the ST225/75R15 Load Range E rating is determined

Tire manufacturers test each tire size and load range combination under controlled conditions to find the maximum safe load and pressure. For the ST225/75R15 Load Range E, that testing shows the tire can carry 3,520 pounds when inflated to 80 psi. The Load Range letter (A through F) is a shorthand for ply rating — the number of fabric layers in the tire carcass. Load Range E typically corresponds to 10 plies, meaning the tire has 10 layers of reinforced fabric woven into its structure. More plies mean a stiffer sidewall that resists flexing under heavy load.

The 80 psi maximum pressure is not a recommendation; it is the pressure at which the tire reaches its rated load capacity. Running the tire at lower pressure reduces how much weight it can safely carry. A tire inflated to 60 psi, for example, cannot carry the full 3,520 pounds even though it is the same tire. Trailer manufacturers specify which load range and pressure combination your trailer needs based on its empty weight and intended cargo capacity. If your trailer documentation calls for ST225/75R15 Load Range E, that is the minimum you should use.

Checking and maintaining pressure in Load Range E tires

Trailer tires need pressure checks before every trip because they sit stationary between uses and can lose air slowly through the valve stem and sidewall. A tire that was at 80 psi last month may be at 70 psi now, which means it is no longer carrying its rated load. Use a quality tire gauge — not the cheap stick gauges that come on air pumps — and check pressure when the tire is cold, meaning at least three hours after the trailer has been parked or before you have driven more than a mile.

Inflate to the full 80 psi even if the trailer is lightly loaded. The pressure rating is not about the load you are carrying today; it is about the load the tire is built to handle. Running at 80 psi keeps the sidewall stiff and the tire cool. Underinflated trailer tires flex excessively, generate heat, and fail suddenly. Many trailer blowouts happen not because the tire was defective but because the owner had not checked pressure in weeks and the tire was running 10 or 15 psi low.

Load Range E versus lower load ranges for the same tire size

The ST225/75R15 size comes in Load Range C, D, and E versions. Load Range C carries 2,540 pounds per tire (10,160 pounds for four tires), Load Range D carries 2,930 pounds per tire (11,720 pounds for four tires), and Load Range E carries 3,520 pounds per tire (14,080 pounds for four tires). The difference is in the number of plies and the thickness of the rubber compound. A Load Range C tire looks identical to a Load Range E tire, but the E version has thicker sidewalls and more internal reinforcement.

If your trailer is rated for Load Range E, do not substitute a Load Range D or C tire to save money. The lower load range tire will fail under the weight your trailer is designed to carry. Conversely, if your trailer is rated for Load Range C, you can use a Load Range E tire without harm — it straightforward costs more and provides capacity you do not need. Always match or exceed the load range specified in your trailer's documentation or on the tire placard mounted on the trailer frame.

When to replace ST225/75R15 Load Range E tires

Trailer tires wear differently than car tires because trailers do not brake — the wheels just roll. This means the tires wear evenly across the tread and last longer in terms of miles, but they age faster in terms of years. Rubber degrades from sunlight, ozone, and heat even when the tire is not being used. Most tire manufacturers recommend replacing trailer tires every five to seven years regardless of tread depth, and many insurance companies will not cover blowout damage if the tire is older than seven years.

Check the tire's age by looking at the DOT code on the sidewall. The last four digits are the week and year of manufacture — for example, 2419 means the 24th week of 2019. If your tires are older than five years and you use the trailer regularly, plan to replace them. Also replace any tire that has visible cracks in the sidewall, chunks missing from the tread, or a bulge in the sidewall. A bulge means the internal structure has failed and the tire will blow out, possibly without warning.

Matching wheel size and load range to your trailer

Your trailer's documentation or the tire placard on the frame will specify the exact tire size and load range required. Do not guess or assume. If you cannot find the documentation, contact the trailer manufacturer with your trailer's VIN (vehicle identification number), which is usually stamped on the frame near the tongue. The manufacturer can tell you the original tire specification and the maximum load capacity of the axle.

The wheel diameter (15 inches in this case) must match your trailer's wheels exactly. A 15-inch tire will not fit a 16-inch wheel, and forcing it on will damage both the tire and the wheel. The width (225 millimeters) and aspect ratio (75) should also match unless your trailer documentation explicitly allows substitution. Load range is the one specification where you can go up — a Load Range F tire will work on a trailer rated for Load Range E — but never go down.

Frequently Asked Questions

Can I use a car tire or light truck tire on my trailer instead of an ST tire?

No. ST tires are built with stiffer sidewalls and different tread patterns designed for the rolling-only stress of trailers. Car and light truck tires are designed for vehicles that brake and steer, and they will overheat and fail on a trailer. Always use an ST tire on a trailer axle.

What does the 75 in ST225/75R15 mean?

The 75 is the aspect ratio, which is the sidewall height as a percentage of the tire width. So a 225-millimeter-wide tire with a 75 aspect ratio has a sidewall height of 169 millimeters (225 × 0.75). A lower number like 70 means a shorter, stiffer sidewall; a higher number like 80 means a taller, more flexible sidewall.

If I only carry light loads, can I use a lower load range to save money?

You should use the load range specified for your trailer, even if you do not always fill it to capacity. The load range rating is built into the tire's structure and pressure requirements. Using a lower load range creates a safety margin that disappears if you ever load the trailer closer to its rated capacity or if you miscalculate the weight.

How often should I replace the valve stems on my Load Range E tires?

Valve stems should be replaced whenever you replace the tire, and they should be replaced separately if they crack or leak. A leaking valve stem will cause slow pressure loss that you might not notice until the tire is dangerously underinflated. Rubber valve stems typically last three to five years; metal stems last longer.

Does Load Range E mean the tire is safe at any pressure up to 80 psi?

No. The tire is rated to carry its maximum load only at 80 psi. At lower pressures, the tire can carry less weight. Running at 70 psi, for example, reduces the load capacity significantly. Always inflate to the full 80 psi when the trailer is loaded.