A snow thrower impeller assembly (24) includes a mounting slot (84) for a wiper (70). The wiper includes a wiper portion (86) that slides into the mounting slot to mount the wiper to the impeller (54). The wiper contacts an interior wall (56) of an associated impeller housing during rotational operation of the impeller in order to limit a gap (74) between an impeller blade (66) and the interior wall. The wiper can move radially inward and outward to remain in contact with the interior wall without input from the operator. Another embodiment of the impeller assembly includes impeller blades with a first portion (116) and a second portion (118). The second portion extends at a non-zero angle from the first blade portion of the impeller blade.