Most consumers don't understand how important having the proper tire can be-it really can mean the difference between life and death. When choosing replacement tires, it is crucial that they meet or exceed the specifications dictated by the vehicle manufacturer. If planning on going up in size, either on the stock wheel or larger aftermarket wheels, it's good idea to ensure the new size conforms to tolerances set by the manufacturer (see the requirements placard typically located on the vehicle's door jamb). It's also important not to change the rolling diameter too much, because drastic changes can throw off a vehicle's traction control systems, braking and acceleration, even the navigation system. A good rule of thumb is not to exceed plus/minus three percent.
There's a lot more to a tire than its size. While proper sizing is important to make sure the aforementioned systems aren't affected, the following tire ratings are the ones that keep you safe.