Tire pressure is the cheapest maintenance you own

If we could get every customer to do one thing, it would not be buying a better tire. It would be checking the pressures once a month. Under-inflation ruins more tires than potholes, nails and bad driving put together, and it does it so gradually that almost nobody notices until the tire is already finished.

Why a soft tire is worse than it looks

A tire that is ten percent under does not look flat. It looks like a tire. But the shape it holds under load has changed, and now the shoulders are carrying more of the weight than the centre of the tread was designed to share with them.

Two things follow. The shoulders wear down faster than the middle, so the tire reaches the wear bars on its edges while the centre still has tread on it, and you buy a tire early. And the sidewall flexes more on every rotation, which generates heat inside the casing. Heat is what ages rubber and weakens the internal structure. In a Houston summer, on hot pavement, that is not a small effect.

Fuel economy drops too, because a soft tire has more rolling resistance. That one shows up on your fuel receipts rather than on the tire, so it is even easier to miss.

How to check it properly

Check them cold. That means before you have driven anywhere, or at least three hours after you last did. Driving heats the air inside the tire and raises the pressure, so a reading taken after a run across town will look fine when the tire is actually soft.

Use a real gauge. The ones built into forecourt air lines take a beating and are frequently well out. A decent handheld gauge is inexpensive and lives in the glovebox.

Set them to what the vehicle calls for, not to what is written on the tire. This trips people up constantly. The number moulded into the sidewall is the maximum the tire can take, not the pressure your car wants. The correct figure is on a sticker in the driver door jamb, and sometimes inside the fuel filler flap.

• Check cold, once a month
• Include the spare, which is the one everybody forgets
• Use the door jamb sticker, not the tire sidewall
• Recheck after a big temperature swing, because pressure moves with the weather

What the tire looks like when it has been run soft

Run your hand across the tread from the inside edge to the outside edge. If the two shoulders feel noticeably lower than the middle, that tire has spent time under-inflated. If the centre is worn down and the shoulders still have tread, that is the opposite problem and the tire has been run over-inflated.

If one edge alone is worn and the other is fine, that is usually not pressure at all. That is alignment, and no amount of air will fix it.

A note on the dashboard light

Most vehicles now have a tire pressure monitoring system, and it is genuinely useful, but it is a warning device rather than a maintenance schedule. The light typically comes on once a tire is already well down, which means by the time you see it the tire has been running soft for a while.

Treat the light as a problem to investigate, not as your monthly check. And if it comes on and the tire looks fine, do not assume the sensor is faulty. It is usually right.

Heat, cold and towing

Air pressure moves with temperature, roughly one psi for every ten degrees Fahrenheit. In Houston that matters twice a year. A cold snap in January will drop every tire on the vehicle by a few psi overnight, which is why pressure warning lights so often appear on the first genuinely cold morning. Nothing has gone wrong, the air has just contracted.

The reverse catches people out in August. Tires set correctly on a cool morning will read high after an hour on hot pavement, and the temptation is to let air out to bring them back to the number on the sticker. Do not. That figure is a cold pressure, and bleeding a hot tire down to it leaves the vehicle under-inflated once everything cools.

Towing is the other case worth knowing. Many trucks specify a higher rear pressure when loaded or towing than when running empty, and the door jamb sticker often lists both. Running loaded pressures on an empty truck rides hard and wears the centre of the tread. Running empty pressures while towing does the more expensive kind of damage.

If you would rather someone else did it, call in at 6903 Lyons Ave and we will check the pressures at the counter. It takes a couple of minutes and there is no charge for looking. The number is (713) 234-0629.