The minimum required depth for tire tread is 1/16 of an inch. This becomes noticeable when the built-in tire wear indicator — the letters TWI and an arrow -- shows up in the tire tread.
However, if the car often travels on wet or slippery roads, some experts advise replacing tires when the tread wears down to 1/8 of an inch. In addition to low tread depth, there are other indicators that a vehicle is ready for new tires. Cuts or cracks in the tire show damage. A bulging tire indicates weakness and is potentially dangerous. This tire should be replaced as soon as possible. Regular rotation of tires, adequate inflation and proper alignment help tires last longer.