It is also a good idea to have them balanced halfway through their tread life. Tire replacement should be done when the tread has worn down to the tread wear indicator. This wear bar indicator is a strip of rubber that can be noticed between the tread. Tire treads under this limit dips into the tire failure zone and can be subject to a ticket for driving on unsafe tires. When buying just two replacement tires, be sure to put the new ones on the front.
Front tires are subject to higher wear as they are the ones doing the entire turning.