How to Check Tire Pressure Properly

This is the highest value two minutes of maintenance available to a driver, and the most commonly done wrong.

The number is not on the tire

The big number moulded into the sidewall, something like "MAX 51 PSI", is the maximum pressure the tire can safely hold. It is not what your car wants.

The correct pressure is set by the vehicle manufacturer based on the car's weight and suspension. You will find it:

• On a sticker inside the driver's door jamb, visible when the door is open
• In the owner's manual
• Sometimes inside the fuel filler door or the glovebox

It is usually somewhere between 30 and 36 PSI for a passenger car, often higher for trucks, and the front and rear specs can differ. Use the number for each axle.

Inflating to the sidewall maximum is the single most common tire pressure mistake. It gives you a harsh ride, wears the centre of the tread, and reduces the contact patch.

Check them cold

Air expands as it heats. Driving heats tires. A tire that reads 32 PSI in the driveway can read 36 after half an hour on the highway.

"Cold" means the car has sat for at least three hours, or has been driven less than about two kilometres.

If you check a hot tire and set it to the placard number, you have actually underinflated it, because it will drop below spec once it cools. This is why checking at a gas station after a drive across town gives you a worse result than checking at home in the morning.

If you have no choice but to check warm: set it about 3 to 4 PSI above the placard, then recheck cold when you can.

The method

1. Get a decent gauge. A pencil gauge or a digital one costs a few dollars. The gauges built into air hoses at service stations are abused constantly and are frequently well out.
2. Remove the valve cap. Keep it somewhere you will not lose it; it keeps dirt and moisture off the valve core.
3. Press the gauge straight on. Firm, square, no hissing. A hiss means it is not seated and the reading will be low.
4. Read it. Compare to the door jamb number.
5. Adjust. Add air if low. If high, press the small pin in the centre of the valve to release a little, then recheck.
6. Do all four. And the spare, if you have a full size one. The spare is the tire nobody checks and the one you need when something has already gone wrong.
7. Replace the caps.

Why it matters more than it sounds

Braking. An underinflated tire has a distorted contact patch and stops longer. This is the one that counts.

Wear. Underinflation wears both outer edges. Overinflation wears the centre strip. Either way you throw away tread you paid for, and you reach the replacement point early.

Fuel. Underinflated tires have higher rolling resistance. A few PSI low across four tires is measurable at the pump.

Heat and failure. An underinflated tire flexes more in the sidewall, which generates heat. Heat is what destroys tires at speed. Most highway blowouts are underinflation, not a sudden puncture.

The New Brunswick cold snap

Pressure drops roughly 1 PSI for every 5 degrees Celsius.

A set correctly inflated at 15 degrees in October is about 5 PSI low at minus 10 in January. That is enough to trigger the TPMS light and enough to matter.

Check after the first hard cold snap, and again midwinter. Expect to add air. This is normal and not a leak.

Equally, do not bleed pressure out on the first warm spring day. Tires set correctly in the cold will read high in April, and if you let air out to the placard number while warm you will be underinflated the next cold morning.

When low pressure is actually a leak

Topping up one tire repeatedly is not a pressure problem, it is a leak. Common causes are a nail you cannot see, a corroded bead seat on an older rim, or a failing valve stem or TPMS seal, which is common after a few New Brunswick winters.

If one tire keeps losing air, bring it in. We will find the leak properly in a water bath rather than guessing, and a repairable puncture in the tread area is a straightforward fix.