Tire pressure, measured in Pounds per Square Inch (PSI) is a trivial thing to maintain, but one of the most important factors of your car’s operation.
But how much tire pressure do you actually need? The industry standard recommends 32 PSI. But to be sure you have the correct tire PSI for your vehicle, always check the placard inside the driver’s door jam. Filling your tires to the manufacturer’s specifications is always your best bet.
There are many different factors in deciding what the PSI of a tire should be. And once you understand them, overriding the manufacturer’s specifications will grant you increases in different areas of performance. Some cars may specify that you use different PSI’s for front and rear tires, while other manufacturers make different tire sizes carry the same PSI amount. Additionally, your Tire Pressure Monitoring System is one of the main causes for impromptu repair shop trips. Knowing what they mean can be the difference between taking your car into the shop immediately versus giving yourself a grace period.
I’ve worked in a tire shop for many years, and I started out just when TPMS sensors were becoming commonplace in vehicles. I saw a huge increase in customers coming in to have us look at their tires because they saw a TPMS light. Seeing the transition from analog tire systems to digital TPMS systems really cemented the importance of tire pressure for me.
Your Car’s Tire Pressure
Your car’s manufacturer made the decision to specify your car’s PSI by factoring in components like ride height, weight distribution, handling, and fuel economy. This is why the best decision is always to fill your tires to the specified amount placed on the door jamb.
Your door jamb on the driver’s side is packed full of useful information, like total weight, manufacturer date, the location it was made. Among other things, this piece of the car has the manufacturer tire PSI recommendations glued to the side. If for some reason your car doesn’t have a tire PSI sticker, or it’s illegible, refer to your owner’s manual.
The information stamped and printed on your car is important to your tires for many reasons. One reason is that some manufacturers display their own recommended PSI on the side of the tire.
Tire stamped values may greatly differ from the amount printed on the car, which, in turn, can cause issues. For example, some tires say “inflate to 32 PSI” on the side of a tire, when the vehicle manufacturer specifies all tires should be inflated to 34 PSI. While 2 PSi doesn’t sound like much, the difference can be noticeable, whether it be in handling or fuel economy.
Your car’s tire pressure will be unique to your vehicle. Taking blanket recommendations isn’t advisable, since the vehicle manufacturer already specifies it should be. Every mechanic from master tech to lube tech always checks the tire pressures inside the door before setting the pressure. It’s safe, recommended, and fail-proof.
Some manufacturers specify a different PSI for tires located on the rear of a vehicle. There are a few reasons why they might do this. One is because the tires are different sizes, therefore a weight distribution will be needed. If a 16 inch tire is on the front, and a 18 inch on the back, if the tires were inflated to the same PSI, the ride height would be thrown off. This will cause issues in performance areas like drivability and handling.
If your car was made after 2007 it will let you know if the PSI is correct, by using the Tire Pressure Monitoring System. This electronic system reports your tire pressure to the car’s computer on a second-to-second basis. If there are any issues with any tires, your tire light will come on.
Some vehicles allow you to see your tire pressures right on the digital dash display.
Another reason recommended tire pressures might differ, or stagger, is to ensure a long lasting life by distributing wear most efficiently. For example, on the Ford E-150 vans, you’ll see the pressures for the front axle are around 50 psi, and the rear are around 80 psi.
These pressures are recommended, along with quarterly rotations, to max out the life of all 4 tires. Tires that are highly inflated on the back take most wear in the center of the tread. And tires on the front, where the engine is, are underinflated, causing more traction for the extra weight. The front tires take more wear on the edges.
So when the tires are rotated, and the air pressure switched, you’re maxing out the life of each tire by changing the stress points of the tire. The front tires are now overinflated, and the rear tires under inflated.
Your car’s TPMS (Tire Pressure Monitoring System) is a digital solution to an analog problem.
Before TPMS, the only way you knew about a small leak would be to either spot it before it made the tire flat, or walk out to a flat.
Even more importantly, issues with tire pressures could cause a catastrophe while the car is moving. It’s for these reasons that all cars built after 2007 must have a TPMS.
Tire Pressure Monitoring Systems aim to keep tire blowouts and accidents to a minimum. Additionally, a slow leak could leave you stranded. You would not be notified of the leak, until you’re stranded on the side of a highway while other cars pass you by. TPMS systems notify you before an untimely occurrence like this can happen.
I’ve noticed lots of customers complain that their TPMS light is always on and find it to be annoying. But that’s what it’s supposed to do - it’s supposed to be more annoying than the annoyance of being stranded on the side of a highway.
The TPMS is a crucial safety device which monitors any type of pressure loss, and records it with your vehicle’s computer. Some manufacturers (most), place a small sensor inside the tire. The valve core, the object that you attach the air hose to in order to inflate a tire, acts as an antenna.
This antenna sends a beacon to your car’s computer, notifying the computer if the pressure is acceptable or not. These notifications get sent hundreds of times per second.
Each tire has a sensor, and each sensor is registered with the computer by the tire location.
This is how your car’s computer will read your tire pressures, and notify you if any of the pressure’s are too low, or high, by displaying a notification on the dashboard.
Other car manufacturers take an outside-in approach. This approach uses sensors already mounted on the car that monitor wheel speed in revolutions per second.
When a tire becomes under inflated, the wheel speed sensor would measure an increase in revolutions because the tire becomes smaller in diameter.
This is the method used by a lot of imported cars like Mercedes and Infiniti. The wheel speed approach removes the need for in-tire sensors, which can be costly (they start around $125 each).
If a faster than normal, or slower than normal speed is sensed, your dashboard will notify you about the issue by displaying the TPMS light.
Specifying Your Own PSI
Once you are comfortable with setting the correct pressure in your tires, and are looking for performance improvements, it may be time to test out your own pressures.
But beware, adjusting anything more than 5 PSI can be extremely dangerous.
That being said, if you were expecting to go on a long trip and wanted to save on gas, adding 3 or 4 extra pounds to each tire would improve fuel economy. And on the flipside, if you’re tracking treacherous terrain like curved roads and dirt trails, lowering the PSI a couple pounds can improve traction.
But it’s important to know how to add and subtract pressure, before you set out to specify your own values. As a test, try adding just one pound of air to each tire and notice the difference. You’ll probably immediately notice a looser steering wheel, and over time, you’ll see savings on fuel.
While there are many different theories on tire pressures, it’s important to stick to foundations. Utilizing any other methods are considered advanced, and should only be used by driver’s with great experience in maintaining tires. And when in doubt, always use the door jamb information to set your tire pressures.
About THE AUTHOR
Christopher Sparks has been servicing vehicles since 2012. After completing the automotive studies program at Camden County College, he was awarded an Associates's Degree in Applied Science. His first job was a lube-tech at Jiffy Lube, and is currently an independent B-Technician servicing vehicles for the United States Postal Service. Christopher is ASE certified and loves rebuilding engines.Read more about Christopher Sparks