Trailer Tire Speed Rating: Why It Is Usually Lower

What a speed rating actually tells you

A tire speed rating is a letter code that indicates the maximum speed at which the tire is designed to operate safely over its lifetime when properly inflated and loaded. It is part of the tire service description found on the sidewall. For a more detailed explanation, see our guide to tire speed ratings.

The speed rating is not a suggestion; it is a limit. Exceeding it, even for a short time, can cause the tire to overheat and fail. For trailer tires, this limit is often lower than what you might expect from a passenger car tire, and understanding why is essential to safe towing.

Why ST tires have lower speed ratings

The letters ST on a tire stand for Special Trailer. These tires are engineered primarily for load-carrying ability and stability when towed, not for the high-speed cornering and braking that passenger car tires must handle. The construction of an ST tire emphasizes a robust sidewall to resist flexing under heavy loads, and the tread compound is chosen for durability under sustained weight rather than maximum grip at speed.

At higher speeds, a tire flexes more rapidly, generating heat. Passenger car tires are designed with materials and tread patterns that dissipate that heat effectively. ST tires, because they are not intended for sustained high-speed running, have a lower heat threshold. Running an ST tire above its speed rating causes the internal temperature to climb quickly, which can degrade the rubber and lead to tread separation.

This is why ST tire speed ratings are typically lower than those found on comparable passenger or light truck tires. They are built for a specific duty cycle: heavy loads, moderate speeds, and long straight hauls, not for racing down the interstate.

Federal standards for trailer tires

In the United States, trailer tires fall under specific Federal Motor Vehicle Safety Standards. The key standard is FMVSS 119, which covers new pneumatic tires for vehicles with a GVWR above 10,000 pounds, as well as specialty tires such as ST types. FMVSS 119 establishes performance requirements including a high speed performance test.

Meanwhile, FMVSS 139, which sets stringent high-speed performance requirements for radial tires used on light vehicles (passenger cars, SUVs, and light trucks), explicitly excludes special trailer tires. According to that standard, it does not apply to ST tires for trailers in highway service. This means ST tires are not required to meet the same high-speed endurance tests as passenger car tires, which is one reason their speed ratings are lower.

Furthermore, FMVSS 110 permits trailers to be fitted with ST tires as long as they comply with FMVSS 119. This legal framework recognizes ST tires as a distinct category with its own performance envelope.

Reading the speed symbol on an ST tire

The speed rating is shown as a letter in the tire's service description, usually near the size and load index. For example, a tire might read ST225/75R15 LRE, where the speed symbol is the letter at the end of the load index. The letter corresponds to a specific maximum speed, as defined by industry standards. You should always check the sidewall to know your tire's speed limit.

If you are replacing a trailer tire, use the same speed rating as the original equipment or select a tire with a higher rating if you plan to tow at higher speeds. However, keep in mind that the load rating must also be appropriate for your trailer's weight. Our article on tire load index explains how to interpret those numbers.

Heat is the enemy: why exceeding the rating is dangerous

When a tire rolls, its sidewalls flex and its tread deforms. This generates internal friction and heat. The faster you go, the more heat the tire produces. If that heat cannot be dissipated fast enough, the tire's internal temperature rises to a point where the rubber and steel cords begin to separate. This is called a tread separation, and it can lead to a sudden blowout.

Trailer tires are especially vulnerable because they are often loaded near their maximum weight capacity. A heavily loaded ST tire running at highway speeds produces more heat than a lightly loaded passenger tire. The combination of heavy load and high speed is what pushes an ST tire beyond its design limits.

The consequences of exceeding the speed rating are not gradual. A blowout on a trailer can cause the trailer to sway violently, potentially leading to a rollover. It is not worth the risk to try to save a few minutes on a trip.

Choosing the right trailer tire for your towing speed

When selecting tires for your trailer, first determine the maximum speed you will be towing. If you stay below 65 mph (which is typical for many trailers), a standard ST tire with the correct load rating is usually sufficient. If you regularly tow at higher speeds, you may need to consider an LT (light truck) tire, which is designed with a higher speed capability. However, LT tires have different load and inflation requirements, so consult our comparison of ST vs LT trailer tires to understand the trade-offs.

You also need to match the tire's load rating to the weight of your loaded trailer. Using a tire with a lower load rating than needed is always dangerous, regardless of speed. See our guide to trailer tire maintenance for more on keeping your tires in top condition.

Finally, always verify the correct inflation pressure for your trailer tires. Under-inflation increases heat generation even at moderate speeds. Our article on tire pressure basics provides guidance on setting the right pressure.

What to pick for your use

Questions

Can I put passenger car tires on a trailer?

You generally should not. Passenger car tires are not designed for the heavy sidewall loads and sustained straight-line towing that trailer tires handle. Using them could lead to premature failure, especially when overloaded.

Why do ST tires have lower speed ratings than LT tires?

ST tires are engineered for load capacity and stability at moderate speeds, while LT tires are built for higher speed operation with adequate load and inflation margins. The construction and rubber compounds differ to handle the heat generated at speed.

What happens if I exceed the speed rating on an ST tire?

Exceeding the speed rating causes the tire to overheat, which can lead to tread separation, belt breakage, and a sudden blowout. This is especially dangerous when towing because it can cause loss of control.

How do I find the speed rating of my trailer tire?

The speed rating is a letter code in the tire's service description on the sidewall, typically after the load index. It is part of the standardized tire markings. Look for a letter like L, M, or N to see your tire's maximum speed.

Is it safe to tow at 75 mph with ST tires?

It depends on the specific speed rating of your tire. Many ST tires have a speed rating of 65 mph or 75 mph, but you must check the sidewall. Exceeding the rating is never safe, regardless of how confident you are in the tire's condition.