The 3% rule, established by the manufacturer, states that the total diameter of your vehicle's tires should be within 3% of their original factory specification. This is the primary reason for this, but also includes other ways larger (or smaller) tires can affect a vehicle's speedometer reading. Changes to the overall diameter of a tire from its original size can make it inaccurate for speedometer readings and other vehicle characteristics. The rule also applies to tires over 3% shorter than stock, but it's not just big, oversized tires. There are also reasons for caution, as long as research is done, you can easily change to wider tires or different-sized wheels without breaking the rule.