What Does PSI for Your Car Mean?
Roadway Questions

What Does PSI in Tires Mean?

The acronym P.S.I. stands for Pounds per Square Inch.  Generally speaking, it is the unit of measure inside a chamber containing gas. In the case of car tires, its the amount of pressure air exerts when inside of the tire.

That is the technical ‘definition’ if you will. Sooooo, what does it mean for you and your car?

I don’t understand the technical definition of PSI. But what I do understand and have been taught is to have the proper air pressure in my tires;(measured in PSI),  in order for them to last and to ensure safe driving conditions. So that begs the question of; how much air pressure (PSI) should I put in my tires?

What should my tire pressure be?

My dad always taught me that the standard PSI for my car tires was 32 pounds per square inch.  Was that the proper number…maybe. But there are a variety of factors to consider. What type of tire do I have? What type of vehicle am I driving?  For instance, in my Toyota Tacoma, the recommended PSI is 30. All of this can have an impact on what your recommended PSI should be.

PLEASE READ: What to Do When You Have a Flat Tire for a complete guide on what to do if you indeed get a flat tire.

So what is the optimum PSI for your vehicle and where can you find it?

Well, optimum PSI varies from car to car.  So I can’t tell you what your tire pressure should be.  However, I can tell you where to look to find it. There are two places actually.

The first one is your owner’s manual, there are all sorts of useful information in there, and included with that you can find the recommended PSI for your vehicle.

Second, and probably easier to find, is a label on the inside of the door jamb on the driver’s side of your vehicle.  You can find the recommended PSI there as well.

Now, if you look at the sidewall of your tire, it may say that the maximum tire pressure for your tire is 60 PSI (pounds per square inch), and you may think AWESOME! More is better, right? Wrong! Do not fill your tires to the maximum. This will cause several problems.

  1. It will change the way your car handles.
  2. It will decrease the life of your tires.  The tread on top of the tires will wear out much more quickly.

Neither of these options is really desired.

As I mentioned, most vehicles will recommend a tire pressure of between 30-36 psi.

So as we can see, there is a recommended PSI for every vehicle.  But what happens if the tire pressure is to low or the tire pressure is too high.  Let’s take a look.

This article deals with your main running tires, please read over here to find out the air pressure of your spare tire.

What is a dangerously low PSI?

We have already established that the recommended air pressure for most vehicle tires is between 30 and 36 PSI.  So what’s a dangerously low air pressure for your tires?

Anything significantly lower than this would be considered a dangerously low tire pressure.  Just to scare you a bit. Back in April of 2015 a man in Bristol, CT, was charged with negligent homicide after he lost control of his vehicle in a snowstorm.

According to a local news report, three of his four tires were between 21 and 24 PSI, when his recommended PSI was 32. So taking this account as a point of reference a seemingly insignificant amount of 10 PSI meant the difference in this man’s being able to keep control of his vehicle.

 The moral of the story?  Even a small reduction in the air pressure of your tires can be dangerous.  If you notice that one of your tires is a little squishy as you walk up to your car, get some air in it.

But you say “ I have the Tire Pressure Monitoring System (TPMS) in my car.  It tells me when my tires are going low!”  That is the yellow horseshoe-shaped light on your dashboard.

Well, that’s half true.  While the TPMS light does tell you that your tire pressure is low; it doesn’t give you this information until your tire pressure is 25% less than the recommended tire pressure.  That is bad.

The TPMS light is a WARNING light, not a friendly reminder light. Tires can be seriously compromised running even 5% below optimum.  

What causes a tire to lose pressure then?  Normal wear and tear cause a tire to lose air pressure.  On average a tire will lose about one pound per square inch per month.  That adds up.

Let’s look at a hypothetical situation. Let’s say your recommended tire pressure is 32 PSI and you haven’t checked your air pressure for six months.  Using our average your tire is now at 26 PSI; which is roughly 18% lower than your recommended tire pressure.

And that’s just normal driving.  This doesn’t take into account any heavy loads that you may have carried. It also doesn’t take into account the fluctuation in temperature.  Which also has an impact on your tire pressure.

The results of low tire pressure are obvious.  First and foremost you could have a blowout. This is dangerous to you and to others.  An underinflated tire will also wear out improperly; shortening the life of the tire and also possibly contributing to a blowout.

To avoid all of this, check your tires regularly. At least once a month.  Use a tire pressure gauge and not your TPMS light as a guide. This is just a little preventive maintenance that you can do, and it only takes five minutes.  Five minutes is worth it, to avoid a world of hurt.

What Happens if My Tire Pressure gets to High?

Now let’s examine the other end of the spectrum.  What happens if I over inflate my tires or my tire pressure is too high.

I wasn’t sure about this myself, and after doing some research, I found conflicting arguments.  It was the opinion of some that running your tire air pressure a little high, while causing a little bit stiffer ride, improves handling and fuel economy.

On the other hand, just like running your tires to low.  An overinflated tire will wear out more quickly and unevenly.  Think of the middle of the tire tread, the “ top” or “bottom”, if you will; bulging out.

So instead of getting even wear across the whole tire, you only have a small section of tire absorbing the wear and tear of travel.  After a while, that tread won’t be there anymore and your tire is set up to fail.

In line with this, less of your tire is also making contact with the road.  Which in turn leads to less traction when you are driving. What happens when you have less traction?  It’s easier to lose control of your vehicle.

Now having less traction may seem hardly noticeable when driving on dry roads.  But what happens when you get an afternoon rainstorm ( as is common here in Florida…every afternoon during the summer). Less traction will get real dangerous, real fast, when you are dealing with wet roads.

Don’t get me started on winter weather.  Before I lived in Florida, I lived in Maine.  Probably the most important piece of equipment for winter driving was your tires.  Over-inflated tires with less traction on snow-covered roads was not a good combination.

Finally, the last drawback to having over-inflated tires is….

A stiffer ride.  Yes, you may be a little more uncomfortable on your ride.  Not a major safety issue with over-inflated tires, but a side effect nonetheless.

Again, it takes five minutes to check your tires.  Take the time to check your tires and make sure they have the optimum psi that your vehicle recommends.

When Should I check the Air Pressure in my Tires?

Okay, so we have discussed all of the terrible things that can happen to an improperly inflated tire, whether over or under-inflated.  So how do you avoid finding yourself in these situations?

It is very easy…

Do a regular inspection of your tires.  When you are leaving for work in the morning. Make it a habit to do a quick visual inspection of your tire.  Does it look a little low? Make it a point to check the air pressure in that tire or tires.

How Often Should I Check the Air Pressure in My Tires?

TIRE AIR PRESSURE SHOULD BE CHECKED AT LEAST ONCE A MONTH

Yes, once a month, break out the tire gauge that should be in your car and do the five-minute inspection.  Don’t do just one or two tires, go around to all four tires and make sure they have the proper PSI.

How do you check the Air Pressure in your tires?

On a scale of 1 – 10 of difficulty, with 10 being ‘go get yourself a mechanic’, this is a one.  The only tool you need is a tire pressure gauge ( if you haven’t gathered that already).

Here are the steps…

  1. Make sure your tires are ‘cold’ or haven’t been driven on for at least three hours.  Heat actually increases the air pressure inside the tire. Therefore, if you check your tires when they are ‘hot’, you will not get an accurate reading.
  2. Unscrew the valve stem cap and apply the tire gauge to the valve stem
  3. You will hear a little bit of air escaping the tire, that’s okay. It should only take a second for your tire gauge to measure the air pressure.  Don’t hold the tire gauge on the valve stem for too long or you will start to release a significant amount of air from the tire.
  4. Compare that number to the one on the inside of door jamb
  5. If the tire pressure matches your vehicles recommended tire pressure you are good to go; if not, get somewhere you can add air to your tires.
  6. You’re Done!

Final Thoughts…

Your tires are your first and best line of defense for safe driving.  Take the time to make sure they are well cared for. Whether it is your everyday commute or you are getting ready for a long road trip; your tires are carrying you there.  Take care of them.

Safe travels, people.

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