There is provided an apparatus which extends a rear box on a truck. A pair of opposing supplemental sidewalls perpendicularly extend from opposing lateral side edges on the truck tailgate. Thus, when the tailgate is opened, the interior surface of the tailgate becomes a planar extension of the truck bed, which in turn extends the length of the truck bed. At the same time the opposing supplemental sidewalls extend the length of the rear box opposing sidewalls. A supplemental tailgate folds into a superposed position over the interior surface of the tailgate when the tail gate is closed. When the tailgate is opened, the supplemental tailgate opens into a ramp and then is folded to perpendicularly extend from a rear edge of the tailgate to form a supplemental tail gate. Thus, the overall length and volume of the rear box is extended.