Most modern passenger cars come with manufacturers recommended tire pressure of about 35 pounds per square inch or PSI.
Some vehicles require slightly lower or slightly higher pressures. Regardless of what your vehicle requires, most manufacturers fit a sticker on the driver-side doorjamb showing the appropriate PSI for your vehicle. If there is no sticker, your owner’s manual should have inflation amounts listed.
The recommended inflation amount is always based on a cold tire temperature. It is recommended that tires always are kept inflated to the closest value set by the vehicle manufacturer, but most vehicles equipped with a tire pressure monitoring system will alert the driver to values about ten percent less than the specified PSI.
Technically, any PSI below the recommended amount is too low, but a deviation of more than ten percent is considered by most to be too low – for a vehicle requiring tire inflation to 35 PSI, a pressure which reads about 31 or 32 PSI is considered underinflated. Some manufacturers can allow up to twenty-five percent air loss before the TPMS light is illuminated. A tire that shows 20 PSI or lower is usually considered to be flat.
What Affects Tire Pressure?
Besides a nail or other object putting a hole in the tire itself, the two most common issues that affect tire pressure are a slow leak and air temperature.
A slow leak is similar to a hole or a tire puncture, but rather than having a puncture which leads to a completely flat tire in a short amount of time, a slow leak is usually much more drawn out.
It can be the result of an improper seal between the wheel and tire upon application of new tires, damaged wheels, cracking tires, damaged air valve, or a previous puncture that has been repaired poorly or that has come undone. Unlike a common tire puncture, the chances of having to replace the tire because of the amount of damage are much more unlikely.
The more common factor that affects tire pressure is the air temperature. As air warms, it expands. As it cools, it contracts. This is why so many people start their vehicles on a cold morning and find the TPMS system showing low air pressure, even if it has not come on in weeks or months.
This is also why manufacturers and mechanics recommend inflating your tires when they are cool – that is when the vehicle has not been in use recently and has been sitting, not necessarily when the tires are cooled to a certain temperature.
What are the Signs of Low Tire Pressure?
The low tire pressure warning light in the instrument cluster is the most obvious way to know when one or more of your tires is too low. TPMS systems were mandated on all vehicles sold starting in 2008, so it is becoming harder to find one without this equipped.
Not all pre-2008 vehicles had this though, and there are still plenty of vehicles on the road that may not be equipped with TPMS. You will know if your vehicle has a tire pressure system by the flat tire icon that lights up when you turn on the ignition.
If you are still unsure, your owner’s manual will have further information. Regardless of whether or not your vehicle has a TPMS system or not, it is good practice to check your tire pressure twice a month as even a small amount of air loss can have long-term impacts if not rectified.
Fuel Economy
Tires are designed to provide the best possible fuel economy when properly inflated by combining grip with the surface area. Similarly to kicking an inflated ball versus kicking one that is not inflated, tires can bog down a vehicle, causing the engine to work harder to make it move.
A small amount of deflation may not be recognizable when you are driving, but when a vehicle is traveling at high speeds, it can make a large difference.
Some sources even estimate that a loss of up to three miles per gallon could be caused by the under-inflation of tires. This will most likely happen if there is more than one tire that is underinflated at a time.
Tire Wear
Premature tire wear is another sign that your tire pressure is too low. Rather than having a uniform tread across the tire, an underinflated tire will wear down on each side with the tread remaining thicker in the middle.
The opposite will occur with an over-inflated tire where the middle of the tire will show premature balding. This is not always an obvious sign of inadequate tire pressure, especially if the tires were just installed. Even though this process might take over a year, it usually becomes obvious which tires have suffered the most from under inflation.
Stopping Distance
Another sign that your tire pressure might be too low is an increase in stopping distance from what is normal. A keen driver will notice this more than the average commuter as this is also not always the most obvious sign of under inflation, but studies show that stopping distances can be severely inhibited by under-inflated tires. In cases like these, it will become more obvious. The problem becomes even worse during rainy and snowy conditions.
Steering and Driving Feel
Steering and driving feel are the most obvious ways to tell if your tire pressure is too low, especially if you have had your vehicle for a while and know what it is like to drive.
Vehicles were built to handle a specific way under ideal conditions, and low tire pressure affects these conditions negatively. Sloppy handling, pulling to one side, difficulty keeping the vehicle driving in a straight line, not being able to steer precisely, and a generally unsettled ride can all point to low tire pressure.
All of this becomes compounded in adverse weather conditions, and in some severe instances, it can cause the vehicle to lose all traction.
What Can Happen if Your Tire Pressure is Too Low?
Driving with low tire pressure is a fairly common problem, but it is also one of the most dangerous. Two very serious consequences can occur as a result.
Sudden Loss of Control
It has already been mentioned that a vehicle’s handling and steering becomes less effective when low tire pressure comes into play and that poor weather conditions can expose these issues even more.
Sudden loss of control can happen at any time when a vehicle’s tires are under-inflated, and when driving at speed, any loss of traction can cause an accident, even if no signs of irregular handling had been present before. This consequence of low tire pressure accounts for several thousand accidents per year.
Blowouts
Tire blowouts are the most dangerous consequence of low tire pressure, and they are caused by a weakened tire structure or an excessive amount of wear. Low tire pressure, coupled with the weight of the vehicle and heavy tire loads can cause one or more parts of the tire to suddenly fail. Most blowouts usually happen without warning, the main reason they can be deadly.
If a blowout occurs on your vehicle, it is important to stay calm. Usually, you will hear a loud bang, quickly followed by a sudden jerk to one side or the other.
To avoid an accident, grab the steering wheel tightly, briefly press the accelerator to resettle and realign the vehicle, then let it gradually slow down by itself as you pull to the side of the road. Once the vehicle slows to about 20-30 miles per hour, you may slowly press the brake to fully bring the vehicle to a stop.
Final Thoughts
Tire pressure becomes too low when it drops below the manufacturers recommended inflated value, often found on a sticker on the driver’s side doorjamb of a vehicle.
The pressure is measured in pounds per square inch, and tire pressure monitoring systems usually alert the driver to pressures that drop about ten to twenty-five percent below recommended value – usually around 35 PSI.
Checking tire pressure about two times per month can ensure that premature tire wear, increased braking distances, inadequate steering, and accidents are avoided because of low tire pressures.
