If you're going to grab new plugs, you should consider gapping them down to .030". That being said, a cat-back kit alone should not overly effect your truck's fuel economy. In general, fuel economy drops with cat-backs are caused by a heavier foot in order to hear the new exhaust. If you haven't changed your driving habits, you may want to consider other trouble shooting. Call us directly at 1-877-578-2433 for other possible solutions.