When I was home last weekend I caught my brother-in-law checking out the underside of my car. My first thought was that he was cutting my break lines. After I determined that he was only looking at my tires, I was obviously relieved.

After much discussion, he convinced me to do some research and write up a post on what the proper tire pressure should be for a car. Much to my chagrin, it turns out that John was right: if you’ve grown accustomed to inflating your tire pressure to the vehicle manufacturer’s recommended pressure, you’re probably not putting enough air in your tires.

Factory Recommendations

The vehicle’s factory recommended tire pressure is located on the inside of the driver’s side door. The factory recommended tire pressure is a compromise between handling, comfort and efficiency.

The tire’s maximum PSI is located on the side of the tire itself. The PSI listed on the sidewall of the tire is the max cold pressure for the tire carrying the highest (weight) load the tire supports.

What we are considering doing is increasing the tire’s PSI over the number listed on the car and moving it closer to the number listed on the tire itself.

Can we do this?

What Constitutes Over/Under-Inflation?

Over-inflation is if we exceed the Maximum Cold Pressure listed on the sidewall of the tire. Under-inflation is if we do not meet at least the tire pressure recommended on the passenger door.

We want to avoid Over/Under-inflation for a variety of reasons.

Over-inflation of the tire may distort the tread to the point where it bulges like a donut. This distortion will reduce the amount of tire in contact with the road and will increase the wear in the center of the tread. Reducing the tire’s contact patch will also affect the car’s traction and handling.

The more common and serious problem is tire Under-inflation.

• An under-inflated tire increases the tire’s rolling resistance. This reduces fuel economy.
• An under-inflated tire flexes more. This leads to increased and uneven tread wear. It can be expected that tire life decreases about 10 percent for every 10 percent it is under-inflated.
• An under-inflated tire is prone to blowout. As the tire flexes more, it becomes more likely to blow.

The Sweet Spot

The manufacturer of your car makes their tire pressure recommendations based on a variety of factors, not all of which pertain to you at any given time. Do you have new or old tires? New or old suspension? Are you driving on the pot-holed, vehicle swallowing streets of Chicago or are you about to embark on a cross-country trip? Do you like to exceed the speed limit in rainy weather?

Simply put, there is no single tire pressure that works for every one of us and yet the vehicle manufacturer has to put a recommendation on the side of its cars. They formulate this recommendation to satisfy the most customers that they can. Often this means that their recommendation leans toward the low end of the Safety Range because it increases passenger comfort.

PSI variations within the Safety Range will affect your gas mileage, ride quality, and tire life. At lower PSI’s you’ll get a softer ride but lower mileage and faster tread wear.

Increases in tire PSI will result in a rougher ride but will also get you better gas mileage and longer tire life. Higher inflation means that tires wear better and last longer. You are also likely to see improved handling as only the parts of the tire that are designed to touch the road do so.

Your highest fuel efficiency savings are going to be made at the low end of the air pressure spectrum. If the manufacturer of your car recommends a relatively low pressure (say 24 PSI), your biggest savings are going to made by making sure that you don’t go below that number.

You can expect to reduce your fuel economy by 5 percent if your tire pressure drops to 22 PSI. If you’re tire pressure falls to 20 PSI, you can expect your fuel economy to drop by about 15 percent.

Conversely however, as you increase your pressure over 24 PSI, your savings will not be as drastic. Increasing your tire pressure to 30 PSI might only get you an additional 5 percent in fuel efficiency and tire life. Subsequent increases in tire pressure over thirty PSI will produce correspondingly less improvement (because it becomes harder to take more tire off the road, you can only reduce rolling resistance so much).

Newsburglar Bonus Tip!! Remember, miles-per-gallon is a poor measure of fuel efficiency. Increasing your car’s gas mileage from 14 MPG to 17 MPG saves as much gas for a given distance as increasing your mileage from 33 MPG to 50 MPG.

In other words, making sure that your truck or SUV has properly inflated tires saves more energy than replacing an Accord with a Prius.

Negatives associated with inflating your tires to a higher pressure:

• Harsher Ride: Increasing PSI decreases the tires ability to absorb bumps in the road. As a result, these bumps will be transmitted to the suspension components which will be required to work harder to reduce noise and comfort.
• Traction: Increased PSI decreases rolling resistance. If you decrease rolling resistance too much, you’ll see a decrease traction. That’s not so good in adverse conditions.
• Damage: Easier to damage by potholes or debris.

How To

So, now that we know we can safely increase our tire pressure past the number imprinted on the side of the door, how do we go about finding a tire pressure that meets our particular requirements?

I’ll use my car as an example. I’ve been happily riding along at the vehicle manufacturer’s recommendation of 30 PSI for as long as I’ve owned the car. All the while, my tires have a sidewall rating of 44 PSI.

One obvious option is for me to simply increase my pressure to the Max Cold Inflation number imprinted on the sidewall of the tire.

I’m not going to do this for a couple of reasons.

First, air pressure inside the tire is affected by temperature. Typically, tire pressure can change by 1 psi for every 10 degrees Fahrenheit of temperature change. A tire that measures 32 PSI of air at a morning temperature of 70 degrees F will reach 35 PSI if afternoon temperatures reach 100 degrees F. This is true even if the car simply sits in the driveway. If you jump on the interstate, a few miles of friction between rubber and the road will likely increase tire pressure close to 40 psi.

Simply put, higher temperature means increased pressure. In the winter, the pressure difference between a cold tire and a running tire can be as much as 7-10 PSI as the tire heats up.

Even if I make sure that my tires are “cold” (vehicle hasn’t been driven for more than 2km) prior to measuring, what time of day should I measure? Do I have an accurate tire gauge?

These factors mean that even if I wanted to increase my tire pressure to the maximum sidewall PSI, I’m not convinced that I could accurately do it without exceeding the limit.

So I’m going to give myself a little wiggle room and drop my target down a few PSI, to say, 41 PSI. In winter, I’ll drop it down another 4 or 5 PSI, to 37 or so.

Second, I’m going to factor my typical driving conditions into my target tire pressure. Living in Chicago, I don’t do a lot of long distance driving over smooth roads. Chicago’s murder rate isn’t the only thing it has in common with a third world city. The roads are filled with potholes that look like they were created by a meteor shower.

As such, while I might increase my tire pressure if I decide to get on the highway and go for a long trip, I’m going to lean a little bit further towards a comforting ride, at the expense of fuel efficiency.

I’m gonna drop my target pressure towards 39 or 40 PSI.

Hint: Raise your tire pressure gradually until you reach a compromise between fuel efficiency and comfort.

The 30 PSI that I’ve been driving at is ridiculously low. I’m not going to inflate my tires to 60 PSI however. I’m simply looking for a better compromise between comfort and fuel efficiency. I suspect I’ll find the sweet point somewhere between 38 and 42 PSI.

Increasing the tire pressure could lead to a significant increase in mileage depending on speed and the nature/condition of the road surface. Try it today.