Proper air inflation is important to making sure that your tires perform the way they were designed to. Air pressure determines how much a tire's shape deflects, or flexes, as it rolls. This affects how well the tire responds to driver input, its ability to support your car's weight, and how easily the tire rolls.
Correctly inflating your tires is also important for ensuring that the tread comes in contact with the road properly and getting the most mileage out of your tires. Underinflating or overinflating your tires causes some parts of the tread to wear faster than others, which shortens the tire's tread life.
An underinflated tire lacks the structural stability to maintain its shape. The tread can't support your car's weight as effectively and flattens against the road more than normal. Your car presses down harder on the sides of the tire, causing the shoulders and sidewalls to wear out faster than the center of the tread. This also allows the center tread area to lift up slightly, making the tire more susceptible to hydroplaning on wet roads.
Underinflated tires have flexible treads and sidewalls that cushion shocks, vibrations, and impacts to improve ride comfort. However, they are susceptible to tread squirm and don't transmit handling forces as easily, making steering and handling more difficult. They also generate more rolling resistance and require more fuel to move. This causes heat to build up within the tire and puts it at risk of a blowout.
In offroad applications, drivers may intentionally "air down" or deflate their tires to put more rubber in contact with the ground, increasing traction on rugged surfaces. This also lets the tires conform to gritty offroad terrain and avoid cuts or punctures. But on paved roads, where this extra traction and resistance is not needed, low tire pressure simply decreases fuel efficiency, hurts driving performance, and increases the risk of tire failure.
An overinflated tire is very rigid due to the compacted air inside pressing on its walls. Since the tread doesn't flex very well, it can't put as much rubber in contact with the road and has a narrower footprint. This focuses most of the vehicle's weight on the center tread area, causing it to wear out faster than the shoulders. Less tread-to-road contact area also reduces the tire's ability to grip the road and provide traction.
Overinflated tires have stiffer treads and sidewalls that can carry heavier loads without collapsing. The tire reacts quickly to driver input, which may enhance steering and cornering response in racing applications. However, they are just as receptive to shocks and vibrations, causing a much harsher ride and putting more wear on your suspension.
When overinflated, a tire maintains its round shape as it rolls. On a perfectly flat surface, this would be ideal for minimizing rolling resistance and improving fuel efficiency. Realistically, overinflated tires are very susceptible to any road unevenness from normal wear, and are easily ruptured by curb impacts, running over potholes, or other road hazards.
A properly inflated tire distributes load stress evenly across its contact patch, ensuring that the tread wears uniformly and lasts longer. The tire is flexible enough to flatten its tread area against the road and provide traction, but still has the structural support needed to carry your vehicle.
When correctly inflated, a tire retains a fairly round shape to keep rolling resistance and heat buildup at manageable levels. The tread and sidewalls should be stiff enough to respond to driver input, but flexible enough to accomodate typical levels of road wear and cushion impacts. For all these reasons, it's a good idea to check your tires' air pressure regularly.
When checking your tires, it's important to remember that air pressure increases as temperature rises. Your tires' air pressure cannot be accurately checked while they are warm. For the most accurate results, you should check and inflate your tire air pressure in the cool hours of the morning, before you start driving.
You should also note that that there are several different air pressure levels associated with your tire. Make sure to set your tires' air pressure to your vehicle manufacturer's recommendations, not the maximum pressure specifications listed on the sidewall:
Keep in mind that these air pressure levels are given based on cold inflation, meaning that a correctly inflated tire will read with a higher pressure if checked in the afternoon or after driving. This is expected, so do not deflate your tires if they appear to be overinflated when hot. Setting your tires to the recommended tire pressure while they are hot will actually result in them being underinflated.