1. Tread Wear - Proper tread depth is essential to prevent hydroplaning and skidding. The minimum tread depth is 2/32” of an inch (1.6 mm). All General Tires are designed with a tire wear indicator located at the base of the groove that provides a visual indication of whether the tread is worn to 2/32″. When the top of the indicator becomes flush with the rest of the tire's tread, the tire is worn to 2/32″, and thus needs to replace it as soon as possible. Additionally, some General Tires are equipped with a Replacement Tire Monitor which indicates when the tire’s tread has worn. The words “replace tire” will appear on the tire to alert you that it’s time to visit your local tire dealer. If your tires don’t include the Replacement Tire Monitor, you can use a penny to check tread depth. Simply place the penny upside down into a tread groove. If you can see all of Lincoln’s head, it’s time for a new tire. Advanced and unusual wear can reduce the ability of tread to properly grip the road in adverse conditions. Visually check tires for uneven wear, looking for high and low areas or unusually smooth areas and any signs of damage. Motorists should also check sidewalls for gouges, cuts, bulges or other irregularities.

2. Climate – The heat of summer and cold of winter can wreak havoc on your tires. Too much exposure to direct sunlight, UV rays, and/or heat, can cause cracking in your tires. If the vehicle is parked outside in the elements, and is not driven with frequency, this can accelerate the process. Extreme cold can also factor into tire wear. 

3. Tire Age - Tires are designed and built to provide many thousands of miles of excellent service. For maximum benefit, tires must be maintained properly to avoid tire damage and abuse that may result in tire disablement. The service life of a tire is a cumulative function of the storage, stowing, rotation and service conditions, which a tire is subjected to throughout its life (load, speed, inflation pressure, road hazard injury, etc.). Since service conditions vary widely, accurately predicting the service life of any specific tire in chronological time is not possible.
While there may not be a specific tire age for removal from service, General Tire along with other members of the tire and automotive industries recommend that all tires (including spare tires) that were manufactured more than ten (10) years previous be removed from service and be replaced with new tires, even when tires appear to be usable from their external appearance and if the tread depth may have not reached the minimum wear out depth. Vehicle manufacturers may recommend a different chronological age at which a tire should be replaced based on their understanding of the specific vehicle application.

General Tire recommends that you regularly inspect your tires. An inspection of the tires should be incorporated during routine vehicle maintenance procedures. If tire damage is suspected or found, it should be carefully assessed by a trained tire specialist immediately.

Source: General Tire