Cold Drops Tire Pressure 1 PSI per 10°F in Detroit

26.3 psi. That is what a tire set to 35 psi on a warm September afternoon reads on an average January morning in Detroit, if nobody adds air and it leaks the normal 1 psi a month that Les Schwab says tires lose. The federal warning line for that tire is 26.25 psi. So the light comes on. We ran the math for every month from October to February, using the normal lows the National Weather Service publishes for Detroit, so you can see when your own car is likely to light up.

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Why the Cold Turns the Light On

Air shrinks when it cools. Inside a sealed tire that shows up as lower pressure. Consumer Reports puts it at about 1 psi for every 10°F drop in air temperature. Our own gas law math agrees. A tire at 35 psi on a 70°F day reads about 28.4 psi at 0°F. That is a 6.6 psi loss over 70 degrees, a little under 1 psi for every 10.

The light itself is set by federal law. The rule is FMVSS 138. It says the warning must come on no more than 20 minutes after a tire falls 25 percent below the pressure on your door sticker. For a regular car tire the floor is 140 kPa, which is about 20 psi. Consumer Reports notes the system has been required on new cars since September 2007, so almost every car on a Detroit street today has one.

Run the numbers for a common 35 psi sticker. Take off 25 percent and the line is 26.25 psi. That is the latest the light is allowed to come on. A 30 psi sticker puts the line at 22.5 psi.

The Cold Alone Is Rarely Enough

Most advice stops at the cold. It tells you the light came on because it was chilly, and that it will go off when you drive. That skips the useful part. A tire filled to exactly 35 psi on a 70°F day does not reach 26.25 psi until about 23 below zero. The coldest morning at Detroit Metro Airport from 1995 to 2023 was 15 below, on January 16, 2009, in NOAA yearly records. So a full tire does not trip the light from Detroit cold alone.

When the light comes on at 25°F, the tire was already low. The cold just made it show. The table below runs the same math backward. It starts with a 35 psi sticker and a fill on a 70°F day, and asks how cold it has to get before the light must come on.

Already low by	Light must be on at	In Detroit terms
0 psi	23 below zero	Colder than any morning from 1995 to 2023
1 psi	14 below zero	Close to the lows of 2009, 2014 and 2019
2 psi	5 below zero	Reached in 12 of the 29 years from 1995 to 2023
3 psi	5°F	A cold snap in January
4 psi	15°F	Colder than the normal January low of 19°F
5 psi	26°F	A normal December morning
6 psi	37°F	A normal November morning

Those first few psi go quietly. Les Schwab says tires routinely lose about 1 psi a month through the rim edge and the rubber. Fill in September, skip a few months, and you are 4 psi down by January before the weather does anything.

Detroit Month by Month

The normals below are the 1991 to 2020 highs and lows the National Weather Service office for Detroit publishes. The pressure columns are our own math for a tire filled to 35 psi at 74°F, the normal September high, then read on a normal morning in each month.

Month	Normal high	Normal low	Morning psi, no leak	Morning psi, 1 psi a month leak
October	62°F	44°F	32.2	31.2
November	49°F	34°F	31.2	29.4
December	37°F	25°F	30.4	27.7
January	32°F	19°F	29.9	26.3
February	35°F	21°F	30.0	25.5

Two things jump out. First, the cold by itself takes about 5 psi off a September fill by January. That is a soft tire, but it is still above the line. Second, add the slow leak and January sits right on the line. By February it is under it.

The daily swing matters too. An October day in Detroit runs from a normal low of 44°F to a high of 62°F. That 18 degree swing is close to 2 psi. That is why the light so often shows up at 7 in the morning and is gone by lunch. The tire did not heal. The sun warmed it.

What to Do When the Light Comes On

1. Look at how it lights. A steady light means low air. A light that flashes for 60 to 90 seconds at start and then stays on means the system has a fault, under FMVSS 138. More on that below.
2. Use a gauge, not your eyes. A tire can be several psi low and still look normal. A pencil gauge costs a few dollars at any parts store.
3. Check it cold. The Tire Industry Association says first thing in the morning is best, or at least two hours after you drive. A reading at the gas station after a drive across town will read high.
4. Fill to the door sticker. The number is on a yellow sticker in the driver door jamb and in the owner's manual, per the same Tire Industry Association page. Front and rear can differ. The number molded on the tire is a maximum, not a target.
5. Drive a little. Consumer Reports says the light often goes off within a mile or so after you top off. Some cars need a reset from the owner's manual.
6. Watch for the odd one out. Cold hits all four tires the same. If one tire is far lower than the rest, or drops again in a week, that is a leak, not the weather.

Set the pressure on a cold morning and the cold is already counted. A tire filled to the sticker at 20°F is right on a 20°F day and a bit high when it warms up, which is safer than the other way around.

Low air also costs gas. Each 1 psi drop in the average pressure of all your tires lowers mileage by about 0.2 percent, per fueleconomy.gov. Four tires 5 psi low cost you about 1 percent, on every tank, all winter.

When It Is the Sensor, Not the Air

If the light flashes first and then stays on, filling the tires will not clear it. The flash is the system telling you it cannot read a tire. On cars with a sensor inside each wheel, the most common cause is a dead sensor battery. DENSO says the batteries are built in with a limited lifespan, and a dead one means a new sensor. DENSO also says there is no way to replace a sensor without special equipment, since the tire comes off the rim and the wheel is balanced again.

A system fault is not an emergency, but it does take away your warning. Until it is fixed, a gauge is your only check, so use it every couple of weeks in winter.

We do not sell tires, and sensor swaps are tire shop work. We do check the tire pressures on every maintenance visit and look underneath at no extra cost while your car sits in your driveway. See our oil and maintenance service and the full price list. For a light that is not about tires, start with our check engine diagnostics. Then read about winter tires on Detroit ice and batteries in a Detroit cold snap.