Load and inflation tables: how pressure buys capacity

A load index is a single point on a curve. It states what the tire may carry at its reference inflation — 35 psi for a standard-load passenger tire, 80 psi for a Load Range E light-truck tire, and so on. Every pressure below that has its own, lower, permitted load, and the relationship is published as a table.

Those tables are where door-jamb placard pressures come from.

What the table looks like

Rows are tire sizes, columns are cold inflation pressures in 5 psi steps, cells are the maximum load in pounds and kilograms. Some manufacturers publish them per size, others per size and load range. The standards organisations publish master versions that most manufacturers reproduce.

Three properties of the curve are worth knowing before reading one:

• It rises with pressure and then stops. Above the reference inflation the permitted load does not keep climbing; the tire's ceiling is its ceiling.
• It is not linear. The first psi buys more capacity at the bottom of the range than at the top.
• It is stated cold. Every figure assumes the tire has not been driven on and is at ambient temperature.

How the placard pressure was chosen

The vehicle manufacturer knows the axle loads at gross vehicle weight. It picks a tire size, reads the table to find the pressure at which that tire carries the required load with margin, and prints the answer on the placard.

That is why placard pressures differ between two vehicles wearing the same tire size, and why the front and rear figures often differ on the same vehicle. The placard is a statement about the vehicle's weight distribution, not about the tire.

It also means the pressure moulded into the sidewall — the maximum — answers a different question entirely. It is the ceiling at which the tire's maximum load applies, not a target.

Substituting a size, which is where the table earns its keep

Change size and the original placard pressure no longer corresponds to a known load. The substitution to work through is:

1. Read the placard: original size, front and rear cold pressures.
2. In the table, find the load the original size carries at those pressures. That is the capacity the vehicle was certified with.
3. Find the pressure at which the new size carries at least that load.
4. Compare that pressure with what the new tire and the wheel permit.

Doing this in the other order — picking a pressure that feels right and hoping the capacity follows — is how a taller, softer-riding tire quietly ends up carrying more than its inflation supports.

The passenger-tire-on-a-truck reduction

A P-metric tire fitted to a light truck, van, or trailer does not carry its full rated load. The standards apply a fixed reduction — the rated capacity is divided by 1.10 — before the tire is compared with the vehicle's requirement.

The reason is duty cycle rather than construction: trucks carry concentrated loads, tow, and run at sustained high loads far more often than the passenger cars the P-metric ratings were derived for. An LT-marked tire in the same nominal size is rated on the truck basis to begin with, which is part of why its load index in the same size is usually higher.

Temperature, and why "cold" is in every sentence

Pressure rises as a tire heats. The widely used approximation is about 1 psi for every 10°F of temperature change, from ambient shifts as much as from driving.

Two practical consequences:

• Checking pressure after a drive reads high, and letting air out to reach the placard figure leaves the tire under-inflated once it cools.
• A seasonal ambient swing of 40°F moves pressures by around 4 psi with nobody touching anything, which is why tire pressure warnings cluster on the first cold morning of autumn.

What the monitoring system is watching

Direct tire pressure monitoring systems on modern vehicles are calibrated against the placard pressure and are required to warn when a tire falls significantly below it — the federal threshold is 25% under. That is a long way down: on a 35 psi placard the light appears somewhere around 26 psi, well below the point at which load capacity has already been reduced.

The warning lamp is a floor, not a gauge.

Front and rear are separate problems

A pickup with a light rear axle unladen and a heavy one loaded is the clearest case, and it is why some placards carry two sets of figures: one for normal use and one for maximum load. The tire does not know which it is carrying, so the pressure has to be set for the load that is actually on it that day.

The same logic applies to a vehicle that spends half its life towing. The tongue weight of a trailer lands on the rear axle, and the load table is where you find what pressure that weight requires — not the placard's unladen figure, which was computed for a different vehicle state.

Where the numbers on this site stop

Every figure computed here is geometry: diameter, width, circumference, revolutions, and the consequences of changing them. Load and inflation are properties of a specific tire's construction and are published by its manufacturer, so they are quoted here from the standard tables (load index, load range letters) and never derived. Your placard and the tire manufacturer's own table are the two documents that decide a pressure.