Tire Pressure and TPMS Basics

Why pressure matters

Tire pressure is one of the few tire-care jobs you can do yourself in a few minutes. It is also one of the most important.

The federal statement vehicle makers must put in owner's manuals says driving on a significantly underinflated tire causes it to overheat and can lead to tire failure. It adds that underinflation also reduces fuel efficiency and tread life, and may affect handling and stopping ability.

Finding the right number

The tire placard

Federal rules require a tire information placard on the vehicle. It is usually on the driver's side B-pillar, and the rules allow other spots near the driver, such as the edge of the driver's door or the forward edge of the rear side door.

The placard is headed 'Tire and Loading Information'. It shows the vehicle maker's recommended cold tire inflation pressure for the front, rear and spare tires, the tire size, and a vehicle capacity weight.

USTMA says the recommended pressure is on a label on the driver's door or in your vehicle owner's manual.

Not the number on the tire

The tire itself also has a pressure figure molded into the sidewall. Continental says that figure is the tire's upper structural limit, not the pressure your vehicle is engineered to run on. Use the placard and manual figure for your vehicle, because that is the one your vehicle maker recommends.

If your tires are a different size

The federal statement in the owner's manual notes that if your vehicle has tires of a different size from the one on the placard, you should determine the proper pressure for them. If you changed sizes, ask the tire seller or tire maker how. You can compare two sizes with our tire size calculator, but it does not give a pressure.

How to check pressure

USTMA says to check tires at least once a month, and to check them when they are cold, meaning parked for at least three hours. The federal statement says to include the spare, if there is one.

A basic tire gauge is enough. Check each tire and compare the reading to your placard number. Add air if a tire is low.

Pressure changes with temperature. USTMA says it rises in warm weather and falls in cold weather by one to two pounds for every 10 degrees. That is why a monthly check, and a check after a big weather swing, is worth the minute it takes.

Driving also raises pressure. Continental says it can raise it by several PSI, enough to skew your readings. If you cannot check cold, for example because you drove to a gas station air pump, treat the reading as a rough guide and check again once the tires are cold.

USTMA's run-flat bulletin also says to check before every long trip. It is a habit worth having on any vehicle.

What TPMS is

TPMS stands for tire pressure monitoring system. Under the federal standard it is a system that detects when one or more of a vehicle's tires is significantly underinflated and lights a low tire pressure warning light on the dash.

The standard applies to passenger cars, multipurpose passenger vehicles, trucks and buses with a gross vehicle weight rating of 10,000 pounds or less, except vehicles with dual wheels on an axle. It was phased in on a schedule. The standard says all vehicles manufactured on or after September 1, 2007 must comply.

Direct and indirect systems

The Tire Industry Association says direct systems use a sensor in each tire that sends a radio signal to a receiver on the vehicle. The sensor is part of the valve stem assembly, and it may or may not be detachable. It also says a few indirect systems use the wheel speed sensors instead.

If your sensors are touched during a tire change, ask the shop how they handle them. USTMA says sensors need appropriate precautions when tires are mounted and removed, and that it is best to follow the system maker's recommendations. Your owner's manual says whether the system needs a reset after a tire change or rotation.

What the warning light means

Federal rules require the low-pressure light to come on within 20 minutes once one or more tires are at or below a threshold. The threshold is 25 percent below the vehicle maker's recommended cold inflation pressure, or a minimum figure set in the standard for the tire type, whichever is higher. The light has to stay on while the condition continues.

The Tire Industry Association describes it as a yellow light shaped like a horseshoe with an exclamation point.

As an illustration only, a placard that says 35 psi would put the trigger at about 26 psi, a long way down from where the tire should be. Your own placard sets your own number.

What to do when it comes on

The federal statement says to stop and check your tires as soon as possible, and inflate them to the proper pressure.

A blinking light

The Tire Industry Association says that if the light blinks for about 90 seconds after the ignition is turned on, and then stays on, the TPMS has a malfunction and needs professional diagnosis. Federal rules describe the malfunction indication as flashing for 60 to 90 seconds and then remaining lit while the problem exists.

TPMS is a backup, not a replacement for a gauge

The federal statement is blunt: TPMS is not a substitute for proper tire maintenance. It is still the driver's responsibility to keep correct pressure, even if underinflation has not reached the level that turns the light on.

The Tire Industry Association and USTMA both say the same thing about monthly checks. Because the light is set to come on at a drop of 25 percent, a tire can be low without any warning.

Special cases

• Run-flat tires require a working TPMS, and USTMA says they still need monthly gauge checks.
• Different tire sizes may call for a pressure other than the placard number, as above.
• New tires should be checked against the size on your sidewall or door placard before you buy.

To find tires for your vehicle, use the tire finder. It starts from your vehicle or the size on your sidewall.