Tires should be replaced based on several factors.

First, the wear of the tire tread is a key indicator. Generally, when the tread depth is less than 1.6 millimeters, it's time to replace the tires. This can be checked with a tread depth gauge.

Second, the age of the tires is important. Even if the tread looks good, tires older than 6 to 10 years may need replacement due to rubber aging and degradation.

Third, damage to the tires such as punctures, bulges, or cracks also calls for replacement.

Regular checks of tire pressure are necessary. If the pressure is consistently low or unstable, it might indicate a problem with the tires.

Also, abnormal noises or vibrations when driving could mean there's an issue with the tires.

To ensure safety and optimal performance, it's recommended to have your tires inspected by a professional at least once a year.

Remember, timely tire replacement not only enhances driving safety but also improves fuel efficiency and handling of the vehicle.