Video: With an exercise as technical and with as many moving parts as the deadlift, there's always something you can do to improve. This video by Eric Cressey explains how the width of your grip can affect your ability to lift that weight. One of the key components of deadlifting is to make sure that your lats (the large back muscles below your shoulder blades) are properly engaged. Doing so keeps your body stable and strong enough so that the weight of barbell doesn't cause your back to round (a big no-no). Eric Cressey, who trains professional athletes at his facility Cressey Sports Performance, advises you to move your hands as close as you possibly can to your thighs while still making sure you end up grasping the knurling (the raised criss-cross pattern) on the barbell.
Too wide of a grip lengthens your bar path, which is jargon for saying that you aren't moving along the path of least resistance for your body. As a result, you waste energy and make things harder than they should be. On the other hand, a wide grip (as in a snatch-grip deadlift) does have its own benefits for building killer grip and back strength, though I wouldn't recommend it until you've got the regular deadlift down pat.
Grip Width for Conventional Deadlift Technique [Eric Cressey]