If you're having to put air in your tires frequently then your tires probably need replacing. Also if the tread on the tires is worn down they should be replaced (a symptom of worn tread is the tires squealing on the road for no apparent reason).
Tires are changed during an auto race because the tires get worn down by the track. It is safer to have good tires than to have tires that are worn down that could blow apart and cause accidents.
Generally you replace them every 30,000 miles. You can also tell if the tread is worn down. You can check the depth of the tread with a penny.
Winter tires should be changed out when the tread has been worn down to around 1.5mm to 1.6mm in thickness, after they have been worn down more they are considered worn out. To measure the tread of ones tires one should use a tire gauge which can be bought at websites such as amazon.
the tyres get worn down and it becomes dangerous because the tyres have no gripBadly worn tires can blowout and you can easily loose control of your vehicle.
Tires have molded indicators called "wear bars." If these are visible across the tread pattern, your tires need to be replaced.
You should get new bike tires when they are worn out, have visible damage, or if you are experiencing frequent flats. It is recommended to replace them every 1,000-2,000 miles or if the tread is significantly worn down.
Worn tires are worn down to 1.6 mm of their remaining tread depth. They could have bulges, bubbles, slits or holes. For safety reasons they should be changed.
If you are wondering how you can repair your worn out auto tires because your tires went flat, then you should check out Repair Your Own Tires Guide. It is online and free.
You tires are worn out because either the car needs an alignment, you are running them with either too low or too high air pressure, suspension parts are worn, steering parts are worn, or the tires were defective.
Too much camber and the tires aren't aligned right so the tires get worn easily
To check if your car tires are worn you want to measure the thread depth on the tires. The thread is the thick part of the rubber made of groves and molding. You want to make sure the thread is still there and there is no splits in the rubber or any metal pieces showing through.you can tell its worn down by looking at the groves in the tire. if they are very thin then its worn down. if they are a nice deep grove then they are in good shape. hope this helps. :)Take a penny and stand it on it's head. When you insert the penny into the tire tread if you can see the top of Lincoln's head the tire is worn out and needs replacing. This means the tire is worn below 2/32" and is a dangerous tire that has little grip and is prone to hydroplaning. If you cannot see the top of his head you the tire is still safe.