Tire Pressure in Cold Weather: Why It Drops, What to Do

Why cold air means a lower reading

A tire is a sealed container of gas. For a gas whose volume stays the same, the pressure is proportional to the temperature, as long as both are counted from the right zero. That is the ideal gas law, and the OpenStax college physics text works through exactly this case with a tire: with no leaks and no change in volume, the final pressure equals the starting pressure times the ratio of the two temperatures.

The catch is the two zeros. The temperature has to be absolute, NASA's Glenn Research Center notes: kelvins in metric units, degrees Rankine in US units, never Celsius or Fahrenheit. And the pressure has to be absolute too. Your tire gauge does not show that. It shows gauge pressure, the pressure above the outside air. OpenStax gives the link: absolute pressure is gauge pressure plus atmospheric pressure, so a tire at 34 psi on the gauge holds about 48.7 psi absolute.

The standard atmosphere is 101,325 pascals according to NIST, which is 14.70 psi. Degrees Rankine are degrees Fahrenheit plus 459.67, and kelvins are degrees Celsius plus 273.15 (NIST). The formula for the cold reading is then:

A worked example with example values: the tires were set to 35 psi on a 70 °F afternoon, and now it is 20 °F.

The absolute pressure fell by 9.4%, the same share as the absolute temperature. But because the gauge leaves out the 14.70 psi of the atmosphere, the reading fell by 4.7 psi, or 13.4% of what the gauge showed before. That is why a cold snap looks bigger on the gauge than the physics alone would suggest. In metric the same example is 241 kPa at 294.3 K becoming about 209 kPa at 266.5 K. For other units, the pressure converter turns psi into bar or kPa and the temperature converter does °F, °C and kelvins.

The formula is an approximation. It assumes no air escapes and that the tire keeps exactly the same volume, the two assumptions the OpenStax example states. It also uses the sea-level standard atmosphere; the outside air pressure where you live may be a little different. For estimating a cold-morning drop the formula is close enough, and the rule of thumb in the next section agrees with it.

From a fall fill to a winter morning

The table applies the formula to tires set on a 70 °F fall day to three example placard pressures, then read on colder mornings. No air is lost, so every psi of difference is the temperature alone. The last column is the drop for the 35 psi tire.

Three things stand out. A tire that was right on a mild fall day can be 4 to 6 psi low on a 10 to 20 °F morning without any leak. The drop grows almost in a straight line with the temperature difference, so you can estimate between the rows. And higher placard pressures lose slightly more psi for the same cold, because the drop is a share of the absolute pressure.

It works the other way in spring. If you top the same tire up to 35 psi on a 20 °F morning, it reads about 40.2 psi on a 70 °F day. That is not a reason to skip the winter top-up; it is a reason to check again when the season turns, which the monthly check below takes care of anyway.

The rule of thumb: 1 psi per 10 °F

You do not need the formula at the pump. A General Motors service bulletin on proper tire pressure, a copy of which is on NHTSA's website, gives the shortcut: generally a 5.6 °C (10 °F) temperature change is equivalent to a 6.9 kPa (1 psi) tire pressure change. Warmer means more pressure, colder means less.

The gas law agrees closely. For the 35 psi example tire around 70 °F, it gives 0.94 psi per 10 °F, or about 6.5 kPa per 5.6 °C. Over the 50 degrees from 70 °F to 20 °F, the rule of thumb predicts 5 psi and the formula gives 4.7 psi. Either is close enough to tell you whether a low reading is the weather or something else.

The rule also helps the other way. If one tire is much lower than the others on the same morning, the cold is not the explanation: all four tires saw the same temperature. NHTSA notes that a tire can suddenly lose pressure if you drive over a pothole or bump into a curb, and recommends keeping a tire pressure gauge in the vehicle.

What “cold” pressure means

The pressure your car maker recommends is a cold inflation pressure. NHTSA defines cold as a tire that has not been driven on for at least three hours. The GM bulletin is a little more practical: a cold tire has been sitting for at least 3 hours, or driven no more than 1.6 km (1 mile). Driving warms the tires, and by the formula above warmer air means a higher reading.

So the best time to measure in winter is before the first drive of the day, or after the car has sat for a few hours. If you can only get to an air pump after driving, NHTSA's advice is to fill to the recommended cold pressure anyway. The tire may then still be slightly under-inflated because of the extra pressure of the warm tire, but NHTSA says it is safer to drive with pressure slightly below the recommendation than with a significantly under-inflated tire. Recheck and adjust once you can take a cold reading.

Do not let air out of a warm tire to bring it down to the placard number. NHTSA tells you to measure cold or to compensate for the extra pressure in warm tires.

A heated garage raises a similar question: tires that sat at room temperature read higher than they will in the cold outside. Neither NHTSA nor the federal standards set an outside temperature for the cold reading. The GM bulletin gives one practical hint for the cold season: if the tire light comes on in cold weather and goes off once the tires warm up, the pressure should be reset to placard pressure at the cold temperature. Read that way, the placard number is what the tires should show when they are cold in the weather you actually drive in.

Where the right number is: the door placard

The right pressure is set by the company that built your car, not the one that made the tires. Federal Motor Vehicle Safety Standard 110 (49 CFR 571.110) requires a placard with the vehicle manufacturer's recommended cold tire inflation pressure for the front, rear and spare tires. Where it goes is spelled out in order:

1. On the driver's side B-pillar, the post behind the front door that you see when the door is open.
2. On cars without that pillar and with two doors on the driver's side that open in opposite directions, on the forward edge of the rear door.
3. If neither works, on the rear edge of the driver's door, and failing that, on an inward-facing surface next to the driver's seat.

NHTSA describes the same places for new vehicles and calls it the Tire and Loading Information label; the recommended pressure is also in the owner's manual. Older cars may have it elsewhere: the GM bulletin lists the driver's side front or rear door edge, the center pillar or the rear compartment lid, and fueleconomy.gov mentions the glove box.

Besides the pressures, the placard must show the seating capacity, the combined weight of occupants and cargo the vehicle should never exceed, and the original tire size, including the spare. The spare can have its own pressure. If no spare is provided, the placard says “none”. Front and rear pressures can differ too; NHTSA says to make sure all tires have the same pressure except where front and rear are supposed to differ, and GM reminds technicians to readjust such cars after a tire rotation.

If your car now runs on a different tire size than the placard lists, the placard no longer covers it. The owner's manual statement that FMVSS 138 requires says that in that case you should determine the proper inflation pressure for those tires. The tire size calculator shows how the two sizes compare; the pressure question goes to the tire or vehicle maker.

Why not the number on the sidewall

Every tire carries a pressure in the small print on its sidewall, and it is tempting to use it in winter to “get ahead” of the cold. Don't. That figure is the maximum permissible inflation pressure, which the federal tire standard (49 CFR 571.139) defines as the maximum cold inflation pressure to which a tire may be inflated. It is a ceiling for the tire, not a recommendation for your car.

The vehicle rules keep the two apart: FMVSS 110 allows a placard pressure other than the tire's maximum only if it is less than the maximum and appropriate for the vehicle's load limits. NHTSA puts it plainly: the correct pressure for your tire is what the vehicle manufacturer has listed, not what is listed on the tire itself. fueleconomy.gov and the GM bulletin say the same.

The right way to handle winter is to bring the cold reading back to the placard number, not to aim above it. If you want to keep track of what you set and when, the car maintenance records guide covers the service log.

What the tire pressure light means

Every passenger car, light truck and van from model year 2008 on has a tire pressure monitoring system (TPMS), NHTSA says, and some 2006 and 2007 models do too. Some systems use sensors inside the tires (direct), others work it out from the wheel speed sensors and other vehicle sensors (indirect). The light is either a tire cross-section with an exclamation mark or a top view of a car with all four tires.

The threshold is set in FMVSS 138 (49 CFR 571.138). The light must come on no more than 20 minutes after a tire's pressure is at or below 25 percent under the vehicle manufacturer's recommended cold inflation pressure, or at or below a minimum activation pressure from the standard's Table 1, whichever is higher. The Table 1 minimums are 140 kPa (20 psi) for standard-load passenger tires, 160 kPa (23 psi) for extra-load passenger tires, 200 kPa (29 psi) for light-truck load range C and 240 kPa (35 psi) for load ranges D and E. The light must stay on as long as the pressure is that low.

How cold would it have to get to trigger the light on its own? The table works it out for standard-load passenger tires filled on a 70 °F day, once exactly to the example placard pressure and once 3 psi low (an example of a tire that was not checked for a while).

Cold alone takes a lot: a 35 psi tire set correctly at 70 °F needs about −23 °F to cross 26.3 psi. But a tire that was already a few psi low in the fall reaches the line on an ordinary winter morning, around 5 °F in the example. That matches NHTSA's description of the light going on and off on cold mornings: marginally low pressure dips below the warning threshold overnight and rises again as the tires heat up through driving or as the day warms. NHTSA's advice is to inspect the tires and check the pressure; the lamp should not come on when the tires are properly inflated.

Two more points from the rules. The TPMS warns only when a tire is significantly under-inflated; the owner's manual statement required by FMVSS 138 says the system is not a substitute for proper tire maintenance, and NHTSA says to keep up the monthly check. And a light that flashes for about a minute and then stays on is the malfunction indicator, not a low-pressure warning: NHTSA describes 60 to 90 seconds of flashing at each start-up until the fault is fixed, and says to contact your dealer for a system inspection. According to the manual statement, the system may then not be able to detect low pressure as intended, and replacement or alternate tires can be one cause.

If the solid light comes on, the required owner's manual text says to stop and check your tires as soon as possible and inflate them to the proper pressure. It also states why: driving on a significantly under-inflated tire causes the tire to overheat and can lead to tire failure, and under-inflation reduces fuel efficiency and tread life and may affect handling and stopping ability.

What the drop costs at the pump

fueleconomy.gov, run by the Department of Energy and the EPA, puts a number on the fuel side: under-inflated tires can lower gas mileage by about 0.2% for every 1 psi drop in the average pressure of all tires. Applied to the example above, a 4.7 psi winter drop on all four tires means about 0.9% lower mileage. For a car that gets 30 mpg (example value), that is about 29.7 mpg, or 0.32 gallons more over 1,000 miles.

It is a small amount, and adding air fixes it. To see your own numbers over a few tanks, the MPG calculator works mileage out from two odometer readings and the gallons you filled.

Does nitrogen help in the cold?

Some tire shops offer nitrogen instead of compressed air. On the temperature effect, the physics gives no reason to expect a difference: the gas law in the first section describes a gas at constant volume and has no term for which gas it is.

What nitrogen is said to change is the slow leak through the rubber. The Government Accountability Office looked at this in its 2007 report on underinflated tires. It notes that nitrogen permeates the rubber more slowly than air and that studies showed nitrogen retains pressure longer and slows tire degradation. It also reports that, according to researchers, no studies had shown the results of nitrogen use on safety and fuel efficiency under normal driving conditions, and that tire researchers and others said either product is effective if drivers check their tires regularly and reinflate when necessary. GM's bulletin adds that its TPMS works correctly with nitrogen in the tires.

So nitrogen does not make the morning drop go away. Whichever gas is in the tires, the fix is the same: check them cold and top them up to the placard.

A winter tire routine

1. Find your numbers once. Read the placard on the driver's door edge or pillar and note the front, rear and spare pressures. Keep a gauge in the car, as NHTSA suggests.
2. Check monthly, cold. NHTSA's interval is at least once a month, all tires including the spare, before driving or after the car has sat for three hours.
3. Add the missing pounds. NHTSA's method: note the difference between the reading and the placard and add that much. Bring the cold reading to the placard number, not to the sidewall maximum.
4. Compare the four tires. The cold lowers all of them by about the same amount. One tire far below the others points to a leak or damage, not the weather.
5. Take the light seriously. A steady light means at least one tire is significantly low: check as soon as possible. A flashing then steady light means a system fault: have it inspected.
6. Check again in spring. Air added on cold mornings reads higher on warm days. The monthly check catches it.

For the conversions along the way, the pressure converter turns a placard in kPa or bar into psi for the gauge at the gas station, and the temperature converter turns a forecast in °C into °F for the rule of thumb.

Frequently asked questions

Sources

• National Highway Traffic Safety Administration (NHTSA), Tires (TireWise): tire pressure steps, monthly check when cold (not driven for at least three hours), Tire and Loading Information label location, not the pressure on the tire, filling a warm tire, TPMS questions and answers
• 49 CFR 571.110, FMVSS No. 110, Tire selection and rims: S4.3 placard on the driver's side B-pillar and alternative locations, recommended cold inflation pressure for front, rear and spare tires; S4.3.4 placard pressure and the maximum permissible inflation pressure (Cornell LII copy of the CFR text)
• 49 CFR 571.138, FMVSS No. 138, Tire pressure monitoring systems: S4.2 warning within 20 minutes at 25 percent below the recommended cold inflation pressure or the Table 1 minimum, S4.4 malfunction indicator, S4.5 owner's manual statement (Cornell LII copy of the CFR text)
• 49 CFR 571.139, FMVSS No. 139, New pneumatic radial tires for light vehicles: definition of maximum permissible inflation pressure, sidewall marking S5.5 (Cornell LII copy of the CFR text)
• General Motors, Service Bulletin 00-00-90-002L, Information on Proper Tire Pressure (January 2022), published by NHTSA: 10 °F (5.6 °C) per 1 psi (6.9 kPa), definition of a cold tire, TPMS in cold weather, TPMS works with nitrogen, do not use the pressure on the tire
• NIST, Guide to the SI (Special Publication 811), Appendix B.9: standard atmosphere 101,325 Pa, psi to pascal 6,894.757, kelvin from degrees Celsius and Fahrenheit
• Urone and Hinrichs, College Physics 2e (OpenStax, Rice University), 11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement
• Urone and Hinrichs, College Physics 2e (OpenStax, Rice University), 13.3 The Ideal Gas Law: tire pressure example, absolute pressure and absolute temperature, constant volume
• NASA Glenn Research Center, Equation of State: the temperature must be an absolute temperature, kelvins or degrees Rankine
• US Department of Energy and EPA, fueleconomy.gov: Keeping Your Vehicle in Shape (about 0.2% lower gas mileage per 1 psi drop in the average pressure of all tires; proper pressure on the door jamb sticker, not the sidewall maximum)
• US Government Accountability Office, Underinflated Tires in the United States, GAO-07-246R (February 2007): compressed air and nitrogen

Written by Tobias. Updated October 22, 2026.