Depending on the ordinances in your state of residence, installing a set of fender flares on your truck could be mandatory if you've installed a wide set of aftermarket tires. To ensure that your flares provide complete tire coverage, you'll need to measure the gap between your fender and your tires.
Measuring For Fender Flares
To find out how wide your fender flares should be, use a tape measure to ascertain how far from your vehicle's fender your tires stick out. Hook the tape measure to the outer edge of your tire, then measure back to the fender and bend the tape measure upward at a 90-degree angle, stopping when the tape is flush with the fender. The measurement displayed at the 90-degree bend is the amount of coverage you'll need. Make sure you choose a fender flare at least as wide as this measurement, if not a little wider.