Cold Pressure, Placard and Sidewall

The recommended cold inflation pressure is on the vehicle tire information placard, usually on the driver door jamb, sometimes in the glovebox or on the fuel filler flap. NHTSA describes that figure as set by the vehicle manufacturer from the vehicle design load limit and its tire size.

The number molded into the tire sidewall is a different quantity. Tires are fitted to more than one vehicle, so the tire manufacturer prints the maximum permissible inflation pressure, which is the most that should ever be in the tire under normal driving.

Under the federal rules, a placard figure below the tire maximum is the ordinary case. Treat the sidewall as a ceiling you stay under, and the placard as the target you set.

Cold means the tire has not been driven on and has not been sitting in the sun. In practice that is first thing in the morning, or after at least a few hours parked in shade.

Driving heats the air in a tire and raises its pressure. If you check at a gas station after a drive and bleed air down to the placard figure, the tire is at the placard figure warm, which means it is below it once cool. You have just underinflated it, carefully.

If you have to set pressure warm, set it above the placard figure by roughly the amount the drive added, then check again cold and correct. There is no way to be precise about that on the forecourt, which is why cold is the instruction.

The rule is not folklore, it is the gas law with the numbers filled in. Pressure in a fixed volume moves in proportion to absolute temperature, and a gauge reads the difference between the inside of the tire and the atmosphere.

Work it at a 35 PSI gauge reading. Absolute pressure inside is about 35 plus 14.7, so 49.7 PSI, and a typical ambient temperature is around 530 degrees Rankine. A ten degree Fahrenheit change is ten parts in 530, so the pressure changes by 49.7 times 10 over 530, which is a little under one PSI.

That is where about one PSI per ten degrees comes from, and the table shows it across a wider swing. It also explains why tires that were fine in September set off warning lights in November: a thirty degree drop is close to three PSI.

The ranges below are the ones the gauge picker uses to work out what scale of gauge suits what you inflate. They are published typical ranges for a class of vehicle, and they exist to size a tool.

They are not a substitute for the placard. Two cars of the same class can specify pressures several PSI apart, and many vehicles specify different front and rear figures, or a separate loaded figure.

Use the range to choose a gauge that puts your actual pressure in a sensible part of its scale. Then use the placard to set the tire.

A tire pressure monitoring system is a warning device. Federal rules require it to alert the driver when a tire is substantially below the placard pressure, which means the tire is already well down by the time the lamp appears.

So the light is not a gauge and its absence is not a report that the tires are correct. A tire can be several PSI low, wearing badly and costing fuel, with nothing on the dashboard.

Some systems display a live pressure per corner, which is genuinely useful and still worth checking against a handheld gauge occasionally, because a sensor is a sensor and they drift and fail like anything else.