How to check tire pressure

Five minutes, once a month, with the car cold and the right number in hand. This is the routine NHTSA describes and the one the Relgard gauge was built for: press, read, compare, top up.

First, find the right number — on the car, not the tire

Open the driver's door and look at the placard on the door jamb. That sticker carries the vehicle manufacturer's recommended pressure for your car, in PSI. Some vehicles specify different pressures for the front and rear tires; the placard says so when yours does. Your owner's manual carries the same figures.

The number molded into the tire's sidewall is not your target. It is the maximum pressure the tire itself can hold. Inflating to the sidewall number overinflates the tire for your car. Placard number for the car; sidewall number is a ceiling, not a goal.

Check cold — the three-hour rule

Pressure readings are only comparable when the tires are cold: the car has been parked for at least three hours, or driven less than a mile at low speed. Driving heats the air inside the tire and pushes the reading up, so a warm tire can look fine while it is actually low — and bleeding air out of a warm tire to hit the placard number leaves it underinflated once it cools. The easy habit: check in the morning, before the car goes anywhere.

The check itself, tire by tire

Unscrew the valve cap and keep it somewhere it cannot roll away.
Press the gauge straight onto the valve stem until the brief hiss stops. With the Relgard digital gauge, the exact PSI appears on the readout; with a pencil gauge you read the stick, with a dial you read the needle — the comparison guide covers what each reading is worth.
Write the number down — four tires is four numbers, and memory blurs them.
Compare each tire against the placard figure for its axle.
Screw the cap back on. The cap keeps dirt and moisture out of the valve.

Reading the pattern, not just the numbers

What you see	What it usually means	What to do
All four down a little, evenly	Weather and time — roughly 1 PSI per 10°F of cooling, plus about 1 PSI a month (AAA)	Top all four up to the placard figure
One tire several PSI below its siblings	Not weather — a slow leak: a nail, a valve, or a rim seal	Top it up, re-check in a few days, and have it inspected if it drops again
One or more tires above the placard figure, checked cold	Overinflation at the last fill	Let air out a little at a time, re-reading as you go
Numbers fine but the car pulls, vibrates, or wears unevenly	Something a gauge cannot diagnose	A tire professional — pressure is one input, not the whole story

Topping up without undoing the check

Note how many PSI each low tire is missing, then add that amount at a service-station compressor or with a home inflator, and re-read with your own gauge rather than trusting the chuck's built-in stick. If a tire needs air again within days, that is the slow-leak pattern above — air is cheap, but a tire that keeps asking for it is telling you something.

When to check

Once a month, on a fixed day you will actually keep.
Before long trips, when the car will run loaded at highway speed.
When the temperature swings hard — the first cold snap of the season is the big one; the cold-weather guide does the math.
When the TPMS light comes on — and after it goes out, because the light only speaks up at 25% low.

The whole routine is four tires in under a minute with a gauge that lives in the glovebox. That gauge is the Relgard Tire Pressure Gauge — $19.97, US shipping included, exact PSI on a digital readout.