All car tires lose air over time, and there are normal and abnormal causes for it.

The normal causes are:

  • Osmosis (permeation).
  • Temperature change.

The abnormal causes are:

  • Wheel parts damage.
  • Tire damage.

It’s important to check your tire pressure once per week/month. When a tire loses more air than 25% below the pressure recommended by the vehicle manufacturer, the tire-caused crash possibility increases threefold. Plus, if you drive on such a tire for some time, you may need to remove it from service as it would be considered flat.

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Normal Reasons Why Car Tires Lose Air Over Time

Osmosis (Permeation)

Osmosis or permeation is the ability of air to pass through the structure of the tire rubber. It exits the tire at the rate of 1-3 psi per month. The exact air loss is usually determined by the model and make of the tire. Different rubber blends provide different rates of permeation.

About 1-3 psi per month is normal for air loss, but you still should check inflation rates often, and re-inflate your tires. Neglecting tire inflation for 6 months may take out 6-18 psi, and with the most frequent recommended rate being 30-35 psi, this is a big loss.

Some drivers prefer to fill their tire casings with nitrogen instead of air. The molecules of nitrogen are bigger, so they penetrate the rubber at a slower rate. However, there are pros and cons to this method.

Nitrogen vs oxygen molecules in tires
Image source: ricksfreeautorepairadvice.com

Temperature Change

Car tires lose air over time when the weather becomes colder – every 10°F drop in temperature removes about 2%. The inflation rate also rises by the same 2% with every 10°F temperature increase. In the US, the change between day and night temperatures may be about 20°F, so tires lose about 4%. So, if your vehicle was outside during a cold night, you may see a decrease by about 1-4 psi, depending on the recommended inflation rate. Don’t inflate the tire more in such a case, as the pressure will return to normal as it becomes warmer and the tire heats.

The theory that car tires lose air over time, due to temperature change, was proven, as shown in these tests.

The key thing to note is the amount of air in the tire is the same, even though the pressure gauge may show otherwise. When the temperature cools down, the air within the tire shrinks. In the same way, when the temperature rises, the air within the tire expands.

In winter though, tires should be re-inflated, as the difference between summer and winter temperatures can be about 50°F. Neglecting tire air pressure checks may sacrifice winter traction and handling.

Abnormal Reasons Why Car Tires Lose Air Over Time

Wheel Part Damage

Wheel part damage may create a hole for the air to escape the tire casing. There are two main kinds of damage that can become air loss causes: a poor valve stem or a bent/damaged wheel.

Poor Valve Stem

Cracked velve stem
Cracked velve stem

Valve stems may deteriorate, due to permanent chemical exposure, promoting air loss. They are the channels that allow us to fill the tire with air or deflate it if necessary. It’s usually recommended to change the valve stems whenever you buy new tires (about every 6-10 years).

However, it can be damaged earlier than that, due to harsh road conditions, hits, and bumps. If you tend to over-tight the valve stem cores after inflating your tires, this may also lead to air leaking. In order to avoid this, keep the torque at about 4 inches per pound. A pre-set wrench can help you maintain this.

So, if you notice your car tires lose air over time at the rate of more than 3 psi a month, you may need to replace the valve stems or their cores.

Bent/Damaged Wheel

A wheel that has partially lost its round shape will not hold the tire tight on it. Thus, the bead area allows some air out of the tire, and it may become flat in a couple of days or a week, depending on the severity of damage. A wheel may bend due to age, or after you hit a road curb or a pothole.

Corrosion is another factor influencing the rate at which car tires lose air over time. Especially if the rim corrodes on the edges where the tire is mounted. Damaged metal will become rusty and may cut the bead or make the surface it sits on become uneven, letting car tires lose air over time. So, if you notice the leak, make sure to run a full diagnosis on your wheels.

Corroded wheel bead seat.
Corroded wheel bead seat.

Tire Damage

Tire damage will let air out directly, no matter how small it is. Some of the most common kinds of damage likely to cause an air leak are:

  • Nails
  • Bead damage.
  • Other road-hazard related damage.

Nails

Nail in a tire
Nail in a tire

A nail in the tire can let out about 2-3 psi daily. Even two days with such an air leak may make the tire’s performance much worse, or even make it dangerous to drive. If the nail stays in the tire tread, the loss of pressure will not be significant, but you shouldn’t leave it like that. While the tire is rotating, more pressure is put on the nail, and it can damage the tire further, causing a blowout.

A puncture from a nail is only reparable if it’s in the tread part of the tire. Any sidewall damage means the tire is no longer fit for purpose, and the repairs should be considered only temporary.

Bead Damage

Damaged tire bead
Damaged tire bead

Even the smallest space from the chunk of rubber in the bead area makes car tires lose air over time. Chunking usually develops as a tire ages, so it’s critical to know the date your tire was manufactured. You can find this out as it’s part of the DOT (Department of Transportation) code on the tire’s sidewall. The last four digits stand for the week and the year of manufacture. You can learn more on date codes from this article.

A piece of debris situated between the bead and the rim, can also let air out. Therefore, when driving on a surface with lots of debris, make sure to inspect your wheels and tires when the drive is over. By removing any foreign objects carefully, you will stop a leak from occurring and avoid rim and/or bead damage.

Other Road Hazard-Related Damages

Hitting a large road curb or pothole may damage a tire. However, multiple hits will most likely make car tires lose air over time. The sidewalls may flex, developing bulges, which means the air is trying to escape through the inner damage. If it succeeds when you hit another road hazard, the tire may blow out.

So, after every minor accident or a curb/pothole hit, check the tire pressure. Also, inspect the tire-wheel unit for any minor damage. Beware of cuts, bulges, and punctures, and if there are none, try to re-inflate the tire. If no air leaks, it means no harm was done. However, if you notice your car tires lose air over time (in a week, for example), consider driving to a tire shop for an inspection.

Tips to Maintain Proper Tire Inflation

These are some things you can do to avoid driving on underinflated tires:

  • Check pressure regularly.
    Some recommend checking pressure once a week, others prefer checking it once a month. You can choose depending on the roads you drive on, your driving style, and the type of your tires. If the roads are harsh and there are many curbs, or if you are a rough driver, inspect the tires more often. Furthermore, sports and performance tires are usually softer and have thinner sidewalls than off-road tires, so they also need more frequent checks.
  • Check air pressure on cold tires.
    All recommendations for tire pressure imply you pump the tires up when they are cold. This means the tires weren’t used in the past two hours (at minimum), or they are cool after a night in a garage. The vehicle also has to be in the shade if you want to check the pressure when the sun’s up. If you check the tires after more than 15 minutes of driving, remove about 1-2 psi from the rate you see.
  • Calibrate your pressure gauge.
    You can give your pressure gauge to a specialist at almost any tire station to calibrate it. This will help you get the most precise reading possible. Every psi is important for the performance and safety of your tires, and even new gauges may show +/- 3 psi before you calibrate it. So, when using a new or poorly calibrated gauge, you may not notice that your car tires lose air over time.

Recommended Tire Pressure Information

You can find the recommended tire pressure in your owner’s manual that comes with your vehicle. Also look at the door jamb to find information placard with tire size and inflation information. Make sure you read the right number, as there are both the recommended pressure and the maximum inflation rate at the maximum load the tires can carry.

Remember, tires will only perform well if they are of the same (or exceeding) load index and speed rating as specified on the placard. You can check all these specs on the sidewall of the tire as a part of its code.

Tire Pressure Monitoring System (TPMS)

Tire Pressure Monitoring System helps determine at what rate your car tires lose air over time. It’s preinstalled in every vehicle manufactured after 2007. The system will tell you whether one of the tires loses air more than the others, and when to check and inflate them. You will see the warning light every time one or all of the tires lose 25% of pressure.

Tire pressure monitoring system alarm
Image source: www.autoserviceprofessional.com

This survey shows how resentful some drivers are towards the TPMS. Only 58% of those surveyed can recognize the waring light of the system. 21% of drivers don’t want to rely on it and prefer to continue driving until they feel the damage themselves. Regardless, this system is recommended and fully approved by the National Highway Traffic Safety Administration (NHTSA).

Our Local Tire Shops in Chicago

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The Importance of Tire Pressure

Some influences of car tires losing air over time are:

  • The tires’ load-carrying capacity.
    It’s not the tires what carries the mass of the vehicle, driver, passengers, etc. It’s the air within them that does, and the capacities are only working if there’s enough pressure in the rubber casing.
  • Tires’ lifespan.
    If there’s always enough air in the tire, it will perform better and last longer. Driving on under- or over-inflated tires causes uneven wear, which that may lead to blowouts, vibration, decreased life expectancy, and other issues.
  • Fuel economy.
    Proper tire pressure will reveal all fuel economy abilities of the tire. When the pressure is out of the ordinary, fuel consumption may increase by about 3%.
  • Your safety.
    Neglecting tire pressure checks causes about 75% of flat-tire issues.

Most passenger vehicles require a tire pressure of about 30-50 psi, while off-road vehicles may need about 80-100 psi. Following the 25% pressure decrease rule, a tire on a light car may become dangerous at about 23-39 psi, while tires on heavy vehicles at 60-75 psi. Considering, at this rate, the tires may be considered dangerous, keep in mind that every 5-7 psi matters for your safety. Therefore, the amount of air within a tire is highly important and needs your attention.