How Load Index Changes with Tire Pressure

How the load index works

Every tire has a load index, a number molded into the sidewall that corresponds to a specific load rating in pounds or kilograms. For example, a tire marked with load index 94 can carry a maximum of 1,477 pounds, and a tire with load index 105 can carry 2,039 pounds. But the number is not absolute: it is the maximum load the tire is rated to carry at a given inflation pressure, as defined in Federal Motor Vehicle Safety Standard No. 109 (49 CFR 571.109).

The same physical tire can carry more weight when inflated to a higher pressure and less weight when the pressure is lower. The load index printed on the sidewall is tied to the tire's maximum permissible inflation pressure, which is also molded into the sidewall, usually 35, 44, or 51 psi for passenger tires depending on the construction. When you run a tire at a lower pressure than that maximum, the load capacity drops accordingly.

This relationship is why a tire can be marked with a load index that seems high enough for your vehicle, yet still be overloaded in practice if the inflation pressure is too low. The vehicle manufacturer's recommended cold inflation pressure, found on the placard in the driver's door jamb, is the pressure at which the vehicle's tires are designed to operate under normal loads. This is separate from the maximum pressure molded on the tire sidewall, which only indicates the upper limit for the tire itself.

The sidewall also tells you how to read the load index and the rest of the tire markings. For a closer look at each marking, see our guide to tire sidewall markings.

Load and inflation tables

The relationship between inflation pressure and load capacity is published in load and inflation tables, which are developed by standards organizations including the Tire and Rim Association (TRA), the European Tyre and Rim Technical Organisation (ETRTO), and the Japan Automobile Tire Manufacturers' Association (JATMA). These tables list, for each tire size and construction type, the maximum load that can be carried at each inflation pressure, usually in increments of 5 psi from about 26 psi up to the maximum.

For a typical P-metric passenger tire, the load table might show a capacity of 1,279 pounds at 30 psi, 1,356 pounds at 32 psi, and 1,433 pounds at 35 psi. The exact values vary by size, load range, and construction. The same tire with a higher load index is not simply stronger at every pressure; it has a higher capacity at every pressure because its internal structure and materials are designed for it.

Light truck (LT) tires use a similar table but with different pressure steps, often starting at 35 psi and going up to 80 or even 110 psi for higher load ranges. The load and inflation table for an LT tire is what a tire dealer uses when determining the correct pressure for a specific axle weight, particularly when the vehicle is used for towing or hauling. You can read more about how load ranges work for light trucks in our guide to tire load range for light trucks.

Why lower pressure means less load capacity

A tire carries load by containing air under pressure. The pressure pushes outward against the tire structure, and the resulting tension in the sidewalls and belts supports the vehicle's weight. When pressure drops, the tire deflects more, the sidewalls flex more, and the internal temperature rises. The load and inflation table accounts for this by reducing the rated capacity at lower pressures; running a tire at a pressure below the one the load index is based on means the tire is carrying more relative to its rated capacity at that pressure.

This is why underinflation and overloading are both dangerous in similar ways. A tire that is overloaded by weight or by low pressure can overheat, which accelerates wear and can lead to tread separation, belt edge separation, or a blowout. The Federal Motor Vehicle Safety Standards recognize this connection. FMVSS No. 110 (49 CFR 571.110), which applies to vehicles with a GVWR of 10,000 pounds or less, requires that the vehicle maximum load on the tire not be greater than the maximum load rating marked on the sidewall, and that the vehicle normal load not exceed 94 percent of the load rating at the recommended cold inflation pressure.

The 94 percent figure in FMVSS No. 110 accounts for normal variations in load distribution and pressure, but it is not a license to run below the recommended pressure. For vehicles above 10,000 pounds GVWR, FMVSS No. 120 (49 CFR 571.120) requires that the sum of the maximum load ratings of the tires on an axle be at least the gross axle weight rating (GAWR) of that axle. These rules are designed to prevent the kind of overload that leads to tire failure.

Dangers of overloading

Overloading a tire beyond the load rating for the actual inflation pressure has multiple consequences. The tire deforms more, generating heat, and the extra heat degrades the rubber compounds and the adhesion between the internal components. Over time, this can cause tread separation, belt edge separation, or a blowout, especially at highway speeds.

Overloaded tires also wear unevenly. The center of the tread may wear faster or slower depending on whether the tire is overinflated or overloaded, and the sidewalls may crack or bulge. In severe cases, the bead area can be damaged, which can allow air to leak and compound the problem.

There is also a regulatory angle. FMVSS No. 110 and No. 120 set minimum tire load requirements that new vehicles must meet, but maintaining that margin is up to you. When you tow a trailer, carry a heavy load, or drive a vehicle that is near its GVWR, the tires need to be inflated to a pressure that supports the actual load. The vehicle placard may list a higher pressure for heavier loads, and for LT tires, the load and inflation table in the vehicle's owner's manual or in the tire manufacturer's data provides the correct value. Our guide on tire load index for towing safety covers this in more detail.

Checking pressure and load

The first step is to check the vehicle placard, usually on the driver's side door jamb or in the glove box, to find the recommended cold inflation pressure for your front and rear tires. That pressure is set by the vehicle manufacturer and corresponds to a load capacity that covers the vehicle's maximum loaded weight, including occupants, cargo, and in some cases a trailer tongue weight.

For normal driving, use that placard pressure. Do not use the maximum pressure molded on the tire sidewall unless you are carrying a load that requires it, and even then, refer to the tire's load and inflation table. The placard pressure is almost always lower than the sidewall maximum, and inflating to the maximum on a lightly loaded vehicle can cause a harsh ride, uneven wear, and reduced traction. For a full breakdown, see our article on tire pressure max vs recommended.

Check pressure at least once a month and before long trips, but always check when tires are cold, meaning the vehicle has not been driven for at least three hours. Tire pressure increases with temperature as you drive, so a reading taken after a long drive will be higher than the cold pressure.

If you are adding a significant load, such as for towing a travel trailer or hauling a heavy payload, look at the load and inflation table for your exact tire size and load range. The table will show the pressure needed to support the heavier load. Some vehicles have a second placard on the door edge or inside the fuel door that lists a higher pressure for full-load conditions. When in doubt, consult the tire manufacturer's published data.

What to pick for your use

The right approach to load index and inflation depends on the vehicle and how you use it. For a daily commuter carrying normal loads, the factory-recommended pressure and standard load tires are sufficient. For a light truck or SUV that is regularly used for towing or hauling, a tire with a higher load index and a higher load range, such as an LT tire, provides more margin, and you need to use the load and inflation table for that tire to set the pressure correctly.

What to pick for your use

Questions

Does a tire's load index change with inflation pressure?

The load index is a fixed number that corresponds to a load rating at the tire's maximum inflation pressure. The actual load capacity at a lower pressure is lower than that rating. The load and inflation table published by the manufacturer shows the capacity at each pressure, and it decreases as pressure drops.

Can I use the maximum pressure on the tire sidewall?

The sidewall maximum is the maximum cold inflation pressure the tire can handle, not a recommended operating pressure. For normal loads, use the pressure on the vehicle placard. Some vehicles list a higher pressure for full-load conditions, and LT tires have load and inflation tables that tell you the pressure needed for heavier loads.

What happens if I overload a tire even though it's at the right pressure?

Overloading makes the tire run hotter, which can cause internal damage, tread separation, and a blowout. It also increases tread wear and can damage the sidewall or bead. FMVSS No. 110 and No. 120 set minimum load requirements to prevent overloading on new vehicles, but you are responsible for staying within your tires' rated capacity.

Do LT tires have different load and inflation tables?

Yes. LT tires use a different table that extends to higher pressures, often up to 80 or 110 psi, and the pressures are not the same as for P-metric tires. The table for a specific LT tire size and load range shows the load capacity at each pressure, and it is what you should use when setting pressure for towing or hauling.

How do I know if my tires are overloaded?

Check the vehicle placard for the recommended pressure and the vehicle's GVWR and GAWR. Compare the load rating of the tires at that pressure against the actual weight on each axle. If the axle weight is near or above the tire's capacity, you need a higher pressure (if the table allows) or a tire with a higher load index.

Is underinflation the same as overloading?

Not exactly, but the result is similar. Underinflation reduces the load capacity so that the same weight becomes a higher percentage of the rated capacity. In effect, a tire that is underinflated for the load it is carrying is overloaded. The load rating defined in FMVSS No. 109 is tied to a specific pressure.