How much does tire pressure drop in cold weather?

How much does tire pressure drop in cold weather?

Tire pressure in cold weather drops by a predictable amount as the outside air cools. Goodyear says tires lose 1 to 2 pounds of pressure for every 10 degrees of temperature drop, and gain the same when it gets hotter. Continental gives 1 to 2 psi, or 0.07 to 0.14 bar, for every 10°C.

The figure follows from the ideal gas law. Inside a tire, the volume barely changes, so the pressure follows the temperature, measured from absolute zero as NASA Glenn Research Center explains. Worked through for a typical car tire at 35 psi, that gives a little under 1 psi per 10°F, or about 1.7 psi per 10°C, just under the low end of Goodyear's range and inside Continental's.

Example calculation: one tire set to 35 psi in a 70°F garage, at sea level, with its volume held constant. Each row is what a gauge would read once the tire has cooled to the outside temperature.

Computed with the ideal gas law (NASA Glenn), constant volume, 14.7 psi atmosphere; close to the 1 to 2 psi per 10 degrees quoted by Goodyear and Continental.

Why does tire pressure drop in cold weather?

Tire pressure drops in cold weather because the air inside the tire contracts as it cools, so the same amount of air pushes on the tire with less force. Nothing has leaked. When the tire warms up again, on a sunny afternoon or after a few miles of driving, the pressure comes back.

The drop shows up overnight. A tire takes on the temperature of the air around it once the car has been parked for a few hours: Kia's owner's manual, for one, counts a tire as cold after at least three hours parked. A car that leaves a warm garage for a freezing street, or sits outside through the first cold night of autumn, meets the full drop the next morning.

Why does the tire pressure light come on when it's cold?

The tire pressure light comes on in cold weather when a tire falls to the warning threshold of the tire pressure monitoring system (TPMS). In the US, federal standard FMVSS 138 requires the light within 20 minutes once a tire is 25 percent below the vehicle maker's recommended cold inflation pressure, or below a minimum of 20 psi for standard passenger tires, whichever is higher. For a placard of 35 psi, the threshold is about 26 psi.

Cold alone rarely gets a correctly inflated tire that far. By the same calculation as above, a tire set to 35 psi at 70°F would have to cool to about -23°F before it reached 26 psi. A tire that started a few psi low gets there on an ordinary cold morning: at 30 psi, the light comes on at about 26°F.

So a pressure light on the first cold morning usually means the tire was already low, and the cold finished the job. That is common: in a 2011 field survey of 6,103 vehicles, NHTSA found that 71.1 percent of model year 2004 to 2011 passenger vehicles had at least one tire underinflated by at least 1 psi, and 12.4 percent had one more than 25 percent low.

The light often goes off after a few miles because driving warms the tires, and a warm tire reads 4 to 6 psi above its cold pressure, per Kia's owner's manual. The light going out does not mean the pressure is right.

Computed with the ideal gas law from the 25 percent threshold of 49 CFR 571.138; constant volume, 14.7 psi atmosphere.

Tire pressure cold vs hot: which reading is right?

The cold reading is the right one. The pressure on the door placard is a cold pressure, which is why the federal standard calls it the recommended cold inflation pressure. Cold means the car has been parked for at least three hours or driven less than 1 mile, in the wording of Kia's owner's manual.

A hot reading runs higher: the same manual says warm tires normally read 4 to 6 psi above the cold pressure, and warns against letting air out of a warm tire to match the placard, because the tire would then be underinflated once it cools. Measured after a drive to the pump, a tire at 39 psi may simply be a correct 35 psi tire that is warm.

What is the right tire pressure in cold weather?

The right tire pressure in cold weather is the same number as in summer: the one on the placard in the driver's door jamb, measured cold. Goodyear and Continental both point to the vehicle maker's recommendation, in winter as the rest of the year.

Two numbers are easy to confuse. The pressure molded on the tire sidewall is the tire's maximum, or rated, inflation pressure, as FMVSS 138 calls it: the most the tire can carry, not a target. And there is no reason to add a few psi on top of the placard for the cold: if the tires are set to the placard while cold, the cold has already been accounted for, and the extra air only overinflates them on the next warm day.

Low tire pressure in cold weather: what to do

Low tire pressure in cold weather is fixed with air, a gauge and the right timing. Here is a plan for a car whose light came on during the first cold snap.

• The morning the light comes on: look at all four tires for one that is visibly lower, then drive gently to the nearest air pump. A tire that looks flat goes on the spare, not to the pump.
• At the pump: measure each tire with a gauge before adding air, and fill to the door placard pressure. The tires have warmed a little on the way, so the reading is not final: never let air out of them here, and recheck them cold the next morning.
• The next morning: measure again before driving, with the tires cold, and correct to the placard. A tire that has lost much more than the others since yesterday may have a slow leak and is worth a shop visit.
• Every two weeks through winter: check the pressures cold, as Continental recommends even for cars with a TPMS. Before a long drive, add a pre-trip tire check.
• When the temperature falls again: each new cold snap takes another psi or two, so a tire set right in October can be low again by January.

Does low tire pressure in cold weather cost fuel?

Low tire pressure costs fuel, a little at a time. fueleconomy.gov says underinflated tires can lower gas mileage by 0.2 percent for every 1 psi drop in the pressure of all four tires. Four tires left 4 psi low after the first cold snap cost about 0.8 percent of fuel economy (example calculation).

That is a small part of the winter loss. A cold engine, cold fluids and short trips cost more, as the guide to why gas mileage gets worse in winter explains, but tire pressure is the part that costs nothing to fix.

How to keep track of tire pressure through the winter

Tracking tire pressure through the winter means writing down four numbers, one per tire, each time you check. A single soft tire is what triggers the light, and an average of four hides it. A note of the date also shows which tire keeps losing more than the others, which is how a slow leak shows itself.

WrenchTime, an iPhone app for car and motorcycle maintenance, records tire pressure on a diagram of the vehicle: front left, front right, rear left and rear right, in the pressure unit you use, with the latest readings on the vehicle card. A tire pressure check is one of its 59 maintenance procedures, so you can schedule a recurring tire pressure check the way you schedule an oil change.

Keep your winter tire checks in one place

Log this winter's readings tire by tire and set the next check before the next cold snap. Download WrenchTime on the App Store: the free version covers 1 vehicle with 2 maintenance entries per month, and WrenchTime Pro removes the limits. Fill-ups and fuel economy sit in the same app, so a winter dip in mileage and a soft tire can be read side by side.