Tonneau covers reduce the amount of aerodynamic drag because the truck bed contains a bowl that can catch a great deal of wind. This will increase the amount of drag, so it's a great reason to buy a softcover as it will pay for itself very fast, although this is not true for hardcovers, so keep that in mind. If you decide to use a softcover variant, make sure it is extra secure; otherwise, it can fly off into the wind.