You should replace your tires based on several key factors:

• Tread Depth: Tires should be replaced when the tread depth reaches the legal minimum in most countries, which is 1.6 mm (2/32 inches), but experts recommend replacing them sooner, around 3 mm (4/32 inches), for better safety, especially in wet conditions. A common test is the Penny Test: if you can see the top of Lincoln’s head when inserted upside down into the tread, it's time to replace the tires. Winter tires should be replaced when the tread is below 4 mm (5/32 inches) for optimal snow traction.
• Age: Regardless of tread depth, tires should generally be replaced every 6 to 10 years because rubber degrades over time, hardens, and becomes brittle, which reduces safety. Checking the DOT code on the tire shows its manufacture date for this purpose.
• Signs of Damage: Visible cracks, bulges, vibrations while driving, frequent loss of air pressure, or poor handling, especially in wet conditions, are indicators that tires should be replaced immediately.
• Mileage and Usage: Tires typically last between 25,000 to 50,000 miles, but this varies with driving habits, tire type, and conditions. Regular tire rotation every 8,000 to 10,000 km helps extend tire life.

Overall, inspecting tires regularly for wear, damage, and age helps ensure safety and performance. Replace all four tires together if possible for balanced handling, but if replacing two, install them on the rear axle to maintain stability. These guidelines help maintain safety, improve fuel efficiency, and prevent accidents on the road.