How many tires do you need?
In ideal circumstances, any time you buy tires, you should replace the entire set. This guarantees optimum matching of brand, type, size, and uniform tread thickness. Some cars and trucks mandate replacement of all four tires at the same time when outfitting your four-wheel-drive (4WD) or all-wheel-drive (AWD) vehicle with fresh rubber.
If you do not replace all four tires at once on an AWD vehicle, you risk damage to components in your AWD system, such as your transfer case. These parts are very sensitive to the overall outside diameter of your tires. Any fluctuation whatsoever can cause damage. Some snow tires should also be replaced with all four at once. This is because the type of tire being installed differs significantly from the construction of your conventional tires and can cause significant handling challenges if mismatched.
For the majority of cars and trucks on the road, it is satisfactory to replace tires at least two at a time. You should replace your worst two tires and keep your newest tires up front. This helps keep the steering accurate, which is important in varying weather and road conditions. Sure, in snowy areas, you may want to put two new tires in the rear if you only replace two because it will aid in severe weather traction. However, this is the exception to the rule. Placing your new tires in the front helps prevent hydroplaning and loss of vehicle control.
When you have a single tire fail, usually by a puncture that is not repairable, there are occasions where it acceptable to replace just one tire. Keep in mind that you want to match both left and right as close as possible, if not identical, in make and model of tire. If your tread thickness or style varies too much, it can cause handling issues with your car and affect vehicle safety.
If you have a full-size spare that has never been used, you can use it with a matching sibling on the other side. In these cases, just make sure the age of the tire is not older than five years, as described earlier.