What Exactly is the Correct Tire Pressure?
by Terry Buis
Owner Sun Valley Automotive
Las Vegas Auto Repair
4553 N. Rancho Dr. Las Vegas, Nevada 89130
I decided to write this Terry’s Tip about correct tire pressure when, a few days ago, I read a question and answer column in a magazine. The reader asked if there was a relationship between tire pressure and fuel economy. The short reply was ‘Yes’ and in the reply the author mentioned that optimal tire pressure can result in a 3% increase in fuel economy and that the correct tire pressure can be found for each vehicle on a sticker inside the driver’s door.
This is a good basic answer, but it’s not completely correct and it’s not the whole story.
Yes, it is true that maintaining the right tire pressure will increase fuel mileage, and it is true that the factory recommended tire pressures for vehicles can be found on driver’s door stickers.
Whether you drive an old classic or a shiny new vehicle, one item of service has stayed constant since the first car rolled of the assembly line – and it applies to every vehicle that rolls down the road supported by tires filled with air – and that is tire pressure, measured in PSI or Pounds per Square Inch.
So where is the fail in this answer about correct tire pressure?
Let me explain in a story…
My once new car is now a few years older, and my tires are showing some age. I start planning ahead, looking at the sales adverts to see where I can get a good deal on a complete set of tires (which do not come cheap these days).
In my search I find a tire dealer who has an okay priced tire set, but he tells me for just a bit more money I can get much better tires – all season, mud and snow with better tread design, stronger compounds and a higher DOT rating. Cool.
The price is not a big jump, and the dealer also offers me a small credit on my old set of tires to re-sell as used tires, which helps to offset the price some. I really like knowing I have a much better set of all around tires….so, I’m sold on the new tires.
They are installed, look great and I’m out the door.
About a year later, I notice one tire looks a bit low compared to the others. I find a gas station that still has a working air fill station to inflate my tire. I asked the guy inside about the correct tire pressure, and he tells me to check the driver’s door sticker for proper inflation – which I do. Then, I air up the tires and off I go.
While getting my oil changed at Terry’s Sun Valley Automotive shop, the technician tells me that the vehicle inspection was okay, nothing needed attention, but mentioned that one tire was a bit low on pressure…there was no sign of damage on the tire, but I needed to keep an eye on it.
I’m thinking, “hmmm, that’s odd” because there was no low tire pressure warning light indicator on my dash…hmm. I told the technician that I noticed it was looking low awhile back, and that I filled it to the factory specs found on the door sticker.
And the technician says, “I see…” and then begins to explain that the tires I have are directional tires – these tires are directionally designed with an arrow indicator molded in the sidewall that indicates the forward directional rotation the tire is to be positioned, a feature found on high quality tires.
Then I’m told that for directional tires, the optimal tire pressure is 44psi cold, not the factory 35psi that is on the inside of my door sticker, which is why my tire is still low on air.
(A brief digression about cold tire pressure…..)
Keep in mind that ‘cold tire pressure’ refers to the actual tire’s temperature. A tire that has been idle for a time or in town driving situations would be considered cold. As a tire builds up speed, heat begins to increase from friction road contact. The rise in heat increases the tire pressure by 5 to 8 psi and is incorporated into the manufacturer’s ‘cold tire pressure’ setting which accounts for the expansion and contraction of tires as the temperature fluctuates.
If you’re crossing the desert on a long run at 80mph, and pull into a gas station, jump out and air up a tire, you would want to compensate for the expansion and contraction of the tire due to hot or cold fluctuations. In this instance, you would not want to add air even when a reading of say 30psi was seen.
Cold tire ratings are the industry standard when checking tires after normal town driving, not after driving on the highway at sustained high speeds.
The Sun Valley Automotive technician explained that it’s a very common mistake, and that if ever in doubt to check the tire specs found on the sidewall of all tires. (It is a government Department of Transportation regulation that all tires must have this information included: on the tire sidewall, you will find the “Tire Manufacturer’s PSI Cold Rating” for that tire).
The technician also told me that there are several types of passenger and light truck psi tire settings. It’s not just the average 35psi correct tire pressure for most tires, some tires can go as high as 80psi for normal tire pressure on passenger truck and trailer tires.
So, in my opinion the author of the magazine column should have explained more to the reader asking about tire pressure and fuel economy. A more thorough and correct response would have said:
35psi is the long standing industry standard for optimal cold tire pressure. But, these days 44psi is also very common, and certain types of tires are rated even higher. So, finding the correct tire pressure should also include checking the sidewall of the tire when in doubt and especially if you purchase high quality or specialty tires.
Take a look at these two pictures below. One pic shows a vehicle’s recommended tire pressure sticker on the inside of the door and the other shows the TMPCR on that vehicle’s tires. As you can see, the tires on the vehicle actually have a higher optimal tire pressure than what is recommended as the average tire pressure for that vehicle.
The industry standard recommended tire pressure is included on the inside of vehicles for the average correct tire pressure on most tires. But, because you can’t send your car to a factory for a new sticker each time you change tire brands and quality, it is also important to check the optimum tire pressure on the tire itself too.
As with most technological advancements, there always seems to be a tradeoff. In this case, depending on the year, make and model of a vehicle, there are also issues to consider when a tire rotation is performed.
For example, on many vehicles (like the Infiniti) the dealer states that once you rotate the tires, the TMPS sensor read out location on your screen will not be correct. If you want to keep the ‘left front’ reading the left front tire, you’ll have to have your computer reprogrammed at a cost of $100 dollars or more. One technical report stated, “The reading position will change as the wheels are swapped positions, but as long as the driver is aware there is no need to reprogram the vehicle each time the tires are rotated.”
We have to be aware on new vehicles today that almost every item on our vehicles is controlled by the computer’s body control module. There are programs that also require computer re-programming once an item is replaced, moved or even serviced. For example, one new LED headlamp on a high-speed Audi costs about $150 dollars for the lamp itself, $400 dollars for the module that died because of age and shorted the lamp, about $150 in shop labor to change out the lamp and install the new module, and another $100 dollars to reprogram the computer so that the lamp and the computer can communicate and work together.
Gone are the days of a #6815 headlight that fit just about every vehicle produced in the last 50 years or so and cost $5.95 plus only took four minutes to install…..and lasted 10 years. Unfortunately, it’s the same today with tires.
Now, here are some Fun Facts and Safety Stuff about Tires!
1. The Tire Pressure Monitoring System (TMPS) reports real time tire pressure information to the driver by a gauge, pictogram visual display or warning light.
2. Fuel economy is affected by underinflated tires by creating a drag or rolling resistance on the roadways. Over the course of a year, a new tire can lose 3 to 9 psi, or about 10% of its correct tire pressure, which equates to about 1% in fuel loss. Add that to a nation of cars in the USA, and this small amount of pressure loss amounts to around 2 billion gallons of gas wasted because of underinflated tires! Hence, the need for TMPS sensors that tell us when our tires are low.
3. Underinflated tires are dangerous to everyone in and around the vehicle. Tires that are underinflated build up excessive heat in the sidewalls and can separate at highway speeds. Statistics show that 44,000 accidents, 33,000 injuries and 650 deaths occur in the U.S. alone every year due to underinflated tires.
Beginning signs of tire separation most of the time appear on the sidewall as small cracks or splits. In our desert climate, this is very common due to the intensity of the sun’s rays and the high temperatures we experience about 8 months out of the year.
Sometimes the cords or steel belts are showing, or in worse cases the sides of the tires are peeling off. But there are also instances when the sidewalls of the tires are okay and do not show any outward signs of problems, but separation to the tires has started internally…perhaps from hitting pot holes or at large rock at some point which caused damage to the membrane of the tire.
Many people just brush off these signs, calling it ‘dry rot’ – but this can be a bad thing to walk away from.
Please take the health of your vehicle’s tires, and the right tire pressure, seriously. Just this past summer on the Las Vegas roadways we’ve had accidents, and some deaths, from tire failure…and this is one roadway hazard that we can all help to prevent.
Even with the right tire pressure, sometimes tires still fail……so remember these Terry’s Tips in case this ever happens (And if you have new teenage drivers in the home, make sure they know these safety tips too).
How to Control a Vehicle if Tire Failures Occurs
Do not immediately steer the vehicle one way or another. Wait a few seconds and let the car stabilize itself while you slowly let off the gas and slowly apply the brake.
Then, slowly begin steering the car off the road.
Notice that everything is S-L-O-W? That is the most important thing to keep in mind if a tire ever goes while you are driving.
When accidents do occur in these cases, they often happen because the drivers over correct, brake too hard or try to get the vehicle off to the side of the road before they have control of the vehicle.
If a tire blows, avoid any sudden hard action. Just get in control of the vehicle first, and then get the vehicle off the road.
Until Next Time, drive safe and check back with
Sun Valley Automotive for more of my Terry’s Tips!
And if you have any comments to add or questions about correct tire pressure that you would like me to answer, send them in through the comment box below.