As we drove past a semi on the highway, my father mentioned that some large trucks have an extra set of wheels. Heavily loaded trailers require additional weight distribution over an increased number of wheels. So, when needed, the retractable set is lowered. When the trailer is empty, however, the additional wheels are raised to improve fuel efficiency and decrease wear and tear on the tires. The wheels are always available, yet their true purpose and value is only revealed when the truck is fully loaded.