Tire pressure in Houston heat

Where the right number is

Your car's pressure is on the tire and loading information placard on the edge of the driver's door or the door jamb, and in the owner's manual. Some cars list a different pressure for the front and rear tires. That placard number is the one NHTSA says to use.

The number molded on the tire's sidewall is the maximum the tire can hold. It is a limit for the tire, not a setting for your car.

How to read the rest of the sidewall

Check it cold

NHTSA counts a tire as cold when the car has not been driven for at least three hours. Driving warms the tire and raises the reading, and so does a car parked in the afternoon sun. In a Houston summer, the cleanest reading is first thing in the morning.

1. Check before you drive in the morning, or after the car has sat for at least three hours.
2. Take the valve cap off and press the gauge straight onto the valve.
3. Compare the reading with the number on the door placard.
4. Add air until it matches, then check it again.
5. Put the valve cap back on.

Never let air out of a hot tire to bring it down to the placard number. Once it cools, it will be low.

What heat does to the reading

Air expands as it warms. A common rule of thumb is that tire pressure moves about 1 psi for every 10 degrees Fahrenheit. A tire set to 35 psi on a mild spring morning will read higher on a summer afternoon, and lower again when the first cold front comes through.

Treat that figure as a rough guide; the sources that publish it do not all state it the same way. The procedure is what matters, and it does not change with the season: check cold, fill to the placard, and do it often.

Why low pressure is the real risk

An underinflated tire flexes more as it rolls. Flexing builds heat inside the tire, and on summer pavement, at highway speed or in stop-and-go traffic, that heat adds up. Heat is what breaks a tire down from the inside and raises the risk of a blowout.

Low pressure also wears the outer edges of the tread faster and makes the car slower to respond. A tire that looks fine to the eye can be well under its number, because a radial tire bulges a little at the bottom even when it is full. A gauge is the only way to know.

Why the light comes on in the morning

The tire pressure monitoring system (TPMS) turns on a warning light when a tire falls well below its set pressure. Pressure is at its lowest when the air is coolest, which is usually early morning. A tire that was already a little low can dip under the warning point overnight, light up on the way out, then warm up and rise back above it. On many cars the light then goes out.

That is why the first cool mornings of fall are when many drivers see the light for the first time in months. Often the tire did not suddenly lose air; the heat was hiding a pressure that was already low.

The light going out does not mean the problem is gone. Check all four tires cold and fill them to the placard.

• One tire low: fill it and watch it. If it is low again in a few days, it probably has a nail or a slow leak.
• All tires near the placard number and the light still on: a sensor or the system may need attention. We service TPMS sensors.
• Light on after a pothole or curb hit: look at that tire and wheel for damage before you drive far.

Flat repair for slow leaks and nails

A routine that works

• Check all four tires once a month, cold, and before any long trip.
• Check the spare too, if your car has one. A flat spare is no help on the shoulder.
• Keep your own gauge in the car, so you can check anywhere.
• After a big swing in the weather, check again.
• If one tire is always lower than the others, have it looked at. A slow leak rarely fixes itself.

A stick gauge or a digital one both work. What matters more is using the same gauge each time, so you are comparing like with like from month to month.

If you would rather not do it yourself, stop in. We will check and set all four, and the spare.