You need to be aware of four things when deciding if you need new tires: tread depth, tread wear, exposure to the elements and the age of the tire.
If you can see the top of Lincolns head it's time for new tires.
With this easy test, a penny can buy you peace of mind when it comes to your tires and safety. Place a penny head first into several tread grooves across the tire. If you always see the top of Lincoln's head, your treads are shallow and worn. If this is the case, your tires need to be replaced.
The most important reason to worry about tread wear is safety. When your tire treads are worn, your vehicle will respond poorly in adverse weather conditions like rain. Make sure your tires are worn evenly from side to side and that they don't have any flat spots or cupping which can keep your vehicle from gripping the road and cause handling problems.
Exposure to heat and the sun's ultraviolet rays may cause structural changes to your tires which can lead to premature failure. This is especially true for vehicles that are parked outside all the time, and especially those that live in the hot desert climates.
How old are your tires? Regardless of tread wear, vehicle manufacturers generally recommend you replace your tires at six years. Why? they dry out over time, they loose their moisture and can become dangerous. Don't risk a tire separation or blow-out, replace your tires if they are over 6 years old.