Does cold weather affect tyre pressure, and by how much?

Does cold weather affect tyre pressure, and how much does it drop?

Tyre pressure in cold weather drops by a predictable amount as the outside air cools. Goodyear says tyres 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 tyre, 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 tyre at 35 psi, that gives about 1.7 psi (0.12 bar) per 10°C, inside Continental's range and close to the figure Goodyear quotes.

Example calculation: one tyre set to 35 psi in a 21°C garage, at sea level, with its volume held constant. Each row is what a gauge would read once the tyre 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 tyre pressure drop in cold weather?

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

The drop shows up overnight. A tyre 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 tyre 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 tyre pressure light come on when it's cold?

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

Cold alone rarely gets a correctly inflated tyre that far. By the same calculation as above, a tyre set to 35 psi at 21°C would have to cool to about -31°C before it reached 26 psi. A tyre that started a few psi low gets there on an ordinary cold morning: at 30 psi, the light comes on at about -4°C.

So a pressure light on the first cold morning usually means the tyre 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 tyre 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 tyres, and a warm tyre 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.

Tyre 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 (1.6 km), in the wording of Kia's owner's manual.

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

What is the right tyre pressure in cold weather?

The right tyre pressure in cold weather is the same number as in summer: the one on the placard in the driver's door jamb or the fuel filler flap, 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 tyre sidewall is the tyre's maximum, or rated, inflation pressure, as FMVSS 138 calls it: the most the tyre 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 tyres 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 tyre pressure in cold weather: what to do

Low tyre 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 tyres for one that is visibly lower, then drive gently to the nearest air pump. A tyre that looks flat goes on the spare, not to the pump.
• At the pump: measure each tyre with a gauge before adding air, and fill to the door placard pressure. The tyres 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 tyres cold, and correct to the placard. A tyre 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 tyre check.
• When the temperature falls again: each new cold snap takes another psi or two, so a tyre set right in October can be low again by January.

Does low tyre pressure in cold weather cost fuel?

Low tyre pressure costs fuel, a little at a time. fueleconomy.gov says underinflated tyres can lower fuel economy by 0.2 percent for every 1 psi drop in the pressure of all four tyres. Four tyres 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 fuel economy gets worse in winter explains, but tyre pressure is the part that costs nothing to fix.

How to keep track of tyre pressure through the winter

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

WrenchTime, an iPhone app for car and motorcycle maintenance, records tyre 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 tyre pressure check is one of its 59 maintenance procedures, so you can schedule a recurring tyre pressure check the way you schedule an oil change.

Keep your winter tyre checks in one place

Log this winter's readings tyre by tyre 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 tyre can be read side by side.