How To Tell If You Need New Tires:
1-Bulges, gouges or cracks
When a tire deflates, it bulges at the sides. This is a sign of low tire pressure or a slow air leak. If you see cracks in the sidewall, gouges or anything that looks odd, take it in to your dealer and have them look at it.

2-Tread wear
One of the best ways to tell if you need new tires is the old-school penny test: Place a penny into the tread, with Lincoln’s head pointing down. If you can still see the top of his noggin, the tread is dangerously low. Most modern tires, however, have wear bars: bits of rubber woven into the pattern at a specific depth. You can find the wear bars that go across the groove, and if the tread pattern is worn down to that wear bar, you need to replace your tire,
Why Worry About Tread Wear?
The most important reason to worry about tread wear is safety. When your tire treads are worn, your car may respond poorly in adverse weather conditions like rain and snow. With good treads your car will grip the road better. Also, having insufficient tread is considered illegal in many states. And finally, worn treads can make other parts of your car wear prematurely.

3-Tire age
How old are your tires? Regardless of tread wear, vehicle manufactures generally recommend you replace your tires at six years. Most tire manufactures recommend you replace your tires at 10 years. Check the manufacture’s recommendations on your specific tires.

4-Tire pressure
Tires deflate at a rate of about one pound per square inch (PSI) per month. You’re not going to wear your tires down in a month, but you should check that you have the right inflation pressure.

5-Temperature Changes
Tire pressure drops in extreme cold. The challenge in summer is excessive heat buildup in the tire. If you’re heading out on a road trip with an underinflated tire, it will generate more heat and wear out faster.

6-Vibration
Watch out for vibration or thumping when you’re driving. If it feels like it’s coming from under the seats, the rear tires may be out of balance. Vibration from the steering wheel could indicate suspension issues. Anything less than a smooth ride means you should take your car in to the dealer.
When it’s time to replace your tires, it is recommended doing so in pairs or all four at a time for better performance and traction and, ultimately, a safer ride.