Tire Load vs Speed: How They Interact

What the load rating means

The federal standard for new pneumatic radial tires for light vehicles defines load rating as the maximum load a tire is rated to carry for a given inflation pressure. The maximum load rating is the rating at the maximum permissible inflation pressure. This definition comes from 49 CFR 571.139, which applies to most passenger vehicles under 10,000 pounds gross vehicle weight rating.

Every tire has a sidewall mold marking that shows its size, load index, and speed symbol. The load index corresponds to a specific load rating at a referenced pressure, but the actual load capacity changes with inflation pressure. The vehicle placard, usually on the driver's door jamb, lists the recommended cold inflation pressure for your vehicle's normal load.

For more detail on load indices, see tire load index rating.

The pressure connection

The relationship is straightforward: as inflation pressure goes up, the load rating goes up, up to the tire's maximum permissible inflation pressure. That maximum pressure is molded on the sidewall. You should never exceed it, because the tire may not be able to contain the stress, especially at higher speeds.

At higher speeds, a tire flexes more and generates more internal heat. If the tire is underinflated for its load, that heat builds up faster and can cause premature failure. Therefore, keeping the tires at the manufacturer's recommended pressure is critical, especially for highway driving.

The U.S. Department of Energy notes that underinflated tires can lower fuel economy, but the primary safety concern is load handling. Proper pressure ensures the tire's footprint is large enough to support the vehicle's weight and dissipate heat. See tire pressure basics for more.

Always use the vehicle's recommended pressure, not the maximum pressure on the sidewall, as a starting point.

Extra load and reinforced tires

Some tires are designed for higher loads and higher inflation pressures than standard tires. 49 CFR 571.139 defines an extra load tire as a tire designed to operate at higher loads and higher inflation pressure than the corresponding standard tire. A reinforced tire is similarly defined as a tire designed to operate at higher loads and at higher inflation pressures.

These tires are often used on vehicles that carry heavy loads or tow trailers. They have a higher load rating at the same pressure, or can achieve the same load rating at a higher pressure. When you choose an extra load tire, you gain extra safety margin for your specific usage.

For a deeper comparison, see Extra Load Tires and Standard Load vs Extra Load Tires.

Speed ratings and load

The speed rating is a separate code that indicates the maximum speed at which the tire can carry its rated load under specified conditions. While these CFR standards do not list speed codes, they include high speed performance tests that tires must pass. For example, 49 CFR 571.109 and 571.119 require that tires meet certain endurance and high speed performance requirements.

In practice, a tire with a higher speed rating is built to withstand more centrifugal force and heat, but it does not automatically have a higher load rating. You must check both numbers. A tire can be overloaded even at moderate speeds if the load rating is insufficient.

When towing or carrying cargo, reduce speed and ensure the tire pressure is set for the expected load. The vehicle manufacturer's placard often provides a higher pressure for heavier loads. See Tire Load Index for Towing and Tire Speed Rating.

Selecting the right tire for your vehicle

The selection process starts with the vehicle's gross axle weight ratings (GAWR). For vehicles over 10,000 pounds GVWR, 49 CFR 571.120 requires that the sum of the maximum load ratings of the tires on an axle be at least the GAWR. For lighter vehicles, 49 CFR 571.110 sets a similar principle: the vehicle maximum load on the tire must not exceed the tire's maximum load rating.

When you buy tires, look at the load index and speed symbol in the tire's service description. Use the vehicle placard to determine the correct inflation pressure and load requirements. If you regularly carry heavy loads, consider extra load tires or tires with a higher load range.

Browse our guide to best light truck tires or best all-season tires for cars and SUVs to find tires that match your load and speed needs.

What to pick for your use

Questions

Does a higher load rating reduce speed capability?

No. Load rating and speed rating are independent. A tire can have a high load rating and a high speed rating. You must respect both for safe operation.

Can I increase load capacity by overinflating?

No. You should never exceed the tire's maximum permissible inflation pressure, which is molded on the sidewall. Overinflation can cause a harsh ride and reduce traction, and it does not safely increase load capacity beyond the design limit.

What is an extra load tire?

An extra load tire is designed to operate at higher loads and higher inflation pressures than a standard tire, according to federal regulations. It offers extra carrying capacity when the vehicle is heavily loaded.

How does speed affect tire load?

Higher speeds generate more heat, which can overstress a tire that is already at its load limit. Maintaining proper inflation pressure and not exceeding the tire's load rating are especially important at highway speeds.

Should I buy a tire with a higher speed rating than my car's top speed?

It is safe to use a higher speed rating, as long as the load rating meets your axle load requirements. A higher speed rating does not mean a higher load rating, so check both.

Where do I find the recommended tire pressure for my vehicle?

Look for the placard on the driver's door jamb or in the glove box. That pressure is recommended by the vehicle manufacturer for normal loads. For heavier loads, you may need to inflate to a higher pressure within the tire's permissible range, as indicated on the placard.