Station wagons, jeeps, and similar motor vehicles which are kept for private use and are designed primarily for the transportation of passengers shall be taxed, rated, and classified on the same basis as other passenger automobiles kept for private use of the same size or weight; and, where used in any statute, rule, regulation, or order, the term “passenger automobile” or “passenger motor vehicle” shall include station wagons, jeeps, and similar vehicles which are kept for private use as passenger cars, unless a contrary meaning is obvious.