There are many aspects to consider when deciding when it is time to buy new tires for your vehicle.  This may not mean that the tread is worn...which is definitely one way to know for certain that that time has come, as well as tires that have been damaged, or have driven for many miles.  You may have purchased new tires for a car, but then perhaps you did not drive the car for many years.  Arid and hot environments will damage the rubber of tires, even when they have not really been taken on the road very often.  There is a code on the side wall of all tires which will give you the information needed to know when exactly that tire was manufactured. Most experienced tire technicians will recommend that if a tire is more than ten years old, regardless of whether or not it has been used, this tire is no longer a safe and viable option for your car.  With that said, checking the tread on your tires can be done either using a tread gauge, or a simple penny.  Check all of the surface area of your tires, as you may not have noticed uneven wear.  Should you discover that the tires have worn more in one spot than in another, this may be evidence that it is time to have the tires balanced, and the front end alignment of your vehicle. And it should go with out saying, that if upon inspection of your tires, should you notice that any elements of the internal structure of the tires is showing through, such as the threads or the mesh, new tires should be the first fix-it you take care of on your vehicle.  Driving through busy city streets, or on back country roads, can become not only inconvenient but dangerous as well, if your tires are not in tip top shape.  This is one of the most preventable 'break down', a blow out in the middle of now where or a blow out during rush hour traffic.  Inspect your tires each time you check the tire pressure, and you will have a better idea of when it is time to make that purchase.