Check tire pressure cold, which NHTSA defines as not driven for at least three hours. The pressure on your door placard is a cold-tire number. NHTSA recommends checking at least once a month, and a check before long trips is a good habit too. Hot tires read higher, so don’t let air out of them to match the placard.
What does “cold” tire pressure mean?
Cold tire pressure is the pressure measured before driving has warmed the tires. NHTSA defines “cold” as a car that hasn’t been driven for at least three hours, which usually means first thing in the morning. It has nothing to do with the weather. A tire can be “cold” on a hot afternoon if the vehicle hasn’t moved.
Sun matters too. A tire sitting in direct sunlight can read higher than the one on the shaded side of the same vehicle. If you can, check them all in the same conditions.
Why do hot tires read higher?
Air expands as it warms, so a tire that has just been driven reads higher than the same tire cold. Driving on pavement heats up the tire from a combination of absorbing the heat of the road, especially on dark colored pavement and the friction heat of the tire on the surface bearing the full weight of your vehicle. The heat raises the pressure inside. For example, if you leave a party ballon in your car in the summer while parked with the windows closed, the heat inside the car will expand the air in the balloon and may even pop the balloon. Similarly, if the balloon is exposed to cold temperatures after being in warm environment, the balloon will shrink.
Outside temperature does the same thing more slowly. AAA notes that tire pressure drops about 1 PSI for every 10°F drop in air temperature. That is why the first cold snap of fall turns on so many TPMS lights.
Should you let air out of hot tires?
No. Don’t bleed air from hot tires to reach the placard number. Once they cool, they could easily be underinflated.
A hot reading above the placard is expected, and it’s built into the spec. The one case where a hot reading tells you something useful is when it’s low. If a tire reads below the placard pressure even when hot, it is definitely low. Add air to the placard number, then recheck it cold later.
How often should you check tire pressure?
Check at least once a month, with the tires cold. That is NHTSA’s guidance on its TireWise page. Beyond that, check before any long trip and at a few other moments:
- Before towing or hauling a heavy load. Your placard and owner’s manual cover loaded pressures, and duallys and RVs should follow the vehicle and tire manufacturer’s load tables.
- When the seasons turn and temperatures swing.
- After airing down off-road and airing back up.
- Any time the TPMS warning light comes on.
- The spare, if you have one. It is the tire everyone forgets, and the placard may list a different pressure for it.
A tire pressure monitoring system (TPMS) is a sensor-based system that warns you when a tire is significantly underinflated. The federal standard (FMVSS No. 138) sets that warning at 25% or more below the placard pressure, so a tire can be noticeably low with no light on. TPMS is a backstop, not a replacement for a gauge.
What’s the right way to check tire pressure?
Use an accurate gauge on cold tires and compare the reading with the vehicle’s Tire and Loading Information placard, which is usually on the driver’s door jamb.
- Find the cold pressure on the placard or in the owner’s manual. Don’t use the “max PSI” on the sidewall; that is a tire limit, not a target. Our door placard vs sidewall guide explains the difference.
- Check before driving, or after the vehicle has sat.
- Remove the valve cap and press the gauge squarely onto the stem until the hissing stops.
- Read the gauge. Add air or let some out until it matches the placard, then recheck.
- Replace the valve cap, and repeat for every tire, including front and rear if their pressures differ.
We make the SKÖG® Platinum Tire Gauge, a battery-free dual-needle dial gauge that lets you set the placard number once and match each tire to it. Any gauge you trust will do the job. See how to use a dual-needle gauge.