An area of confusion for many of us is, "What's the correct air pressure for my tires?" Using the correct air pressure delivers many benefits, including performance and fuel economy. During the rotation of the tire, heat is generated, especially when the leading edge of the tread makes initial contact with the road (deflection). Having too much deflection, for example, an underinflated tire, will create more heat during deflection. This can result in premature failure of the tire. It will also negatively impact steering response and stability while cornering. On the other hand, an overinflated tire exhibits non-desired behaviors as well. Most noticeable would be a harsher ride and an increased risk of tire damage when hitting a pothole. Additionally, the contact patch of an overinflated tire is smaller, which will reduce traction and handling. For a better understanding, read "Air Pressure - Correct, Underinflated and Overinflated."
Where is that sweet spot? There are a couple areas that I see confusion set in when dealing with this topic. The first is that the tire sidewall will state something similar to (see photo below): "Max Load 3195 lbs at 65 psi."
This is telling you the maximum load capacity and air pressure the tire can hold. This is not telling you what air pressure to run, rather it's stating what air pressure not to exceed.
The second area of confusion comes when an alternate tire size is being used. I like to explain this as the air pressure is a volume measurement. As such, you must take into account the volume of the air chamber inside the tire. When a proper plus size is done, you should end up with a very similar sized air chamber. For example, when going from a 17" wheel to an 18" wheel, your tire sidewall will be shorter, but the tire will be wider. Therefore, the net volume loss is near zero. You can read about plus sizing in my previous blog "Plus 1 Plus 2 Equals What?"
Your vehicle dictates air pressure. The mechanical engineers that spent millions designing your vehicle, take everything into consideration. Things like ride comfort, handling and most importantly, how much load capacity the vehicle needs to operate safely. They then print up the door placard and often place it on the driver side door. Again, the tire does not know what vehicle it ends up on, so the tire can't tell you what pressure to use.