Tires are the single most important mechanical component of vehicle control and safe driving. You have only three ways to control a car: brake, accelerate, or steer. All are dependent upon rolling friction with the ground beneath your wheels and the only contact you have is through the tires.
Under normal conditions the contact patch is about the size of your palm. Under heavy cornering, that patch may shrink to the size of your thumbprint. Think about that for a moment and you'll realize the condition of your tires is critical.
There are three circumstances when you should immediately replace a tire.
Stone Bruise: These appear as bubbles in the sidewall and are caused by objects impacting the sidewall of the tire. The thin layers of the sidewall become damaged, air seeps between the damaged layers and soft spots or bubbles build up. These are prone to blowouts.
Tire tread serves two functions. The first is to dissipate heat, much as the fins on a radiator. Tires are formed through a heat process and heat destroys tires.
Cuts: Cuts are caused by sharp objects slicing the sidewall. If you can see the cords (these look like ropes or spaghetti) or they can be felt by probing inside the cut, the tire has been compromised and is prone to blow out.
Tire Tread: Wear Bars will appear when tread depth falls to 2/32 of an inch. This is where the tire tread is worn away and a line appears across the width of the tire. These are designed to tell a driver when the tread depth has fallen below the federal standard.
The second function is to channel water. Without tread the tire rides on top of any water standing on the roadway surface. This is called hydroplaning. (The Indy 500 cars race on slicks, tires without tread. That's why they stop the race at the first raindrop that falls). As we said before, control of the car is dependent upon friction with the ground beneath your wheels.
When you see any of these conditions, the tire needs to be replaced. Under most circumstances, when replacing tires, you should replace all four because all tires are not created equal. Mixing tires of different sizes will unbalance your car's suspension, thereby decreasing the car's stability and adversely affecting your vehicle control. Even if they're all the same size, mixing brands is not prudent. Each model has it's own unique characteristics. Also, if you're going to use studded tires in the winter, you should put studs on all four tires.
Generally, a wider tire provides better traction because there is more rubber in contact with the road.
Tires that are made of hard rubber will wear longer, but do not provide as much traction. Conversely, a softer tire will wear out sooner, but is stickier.
Proper Inflation: A properly inflated tire will be within 1 to 2 pounds + or - of the tire manufacturers recommended max psi. This is printed on the sidewall of your tire. Proper inflation will give the maximum tire life and deliver the maximum handling characteristics engineered into the tire. (Your automobile owner's manual may indicate a lower air pressure for the tires. That is because an under-inflated tire will give a smoother ride)
You should check your tire pressure on a regular basis. On average, a tire will lose one pound of pressure per month. We don't know where it goes. (Perhaps, to the same place as the sock missing from the dryer!)
Under Inflation: An under-inflated tire will appear worn on the outer edges of the tire. This is the most common cause of high-speed tire failure. An under-inflated tire generates tremendous heat. And like we said, heat destroys tires.
Over Inflation: An over-inflated tire will balloon on the road surface. The center of the tire will appear more worn than the edges. It will not corner as well and is more prone to hydroplane. This is because the tread meant to channel the water is not in contact with the road surface.
You can check for most of this yourself by just walking around the car before you get in. (And you won't even have to get dirty) Is it worth the time? You bet your life it is.