Tricky Tonnage by Malcolm Jameson

Elmer Nicklheim was an amateur inventor, and it got to the point folks around town expected the strange stinks coming from the Nicklheim barn. But when Elmer's father died, he seemed to settle down and take up hauling work with his father old truck. But hauling was incidental. The truck was powered by gravity, as part of Elmer's solution to the Unified Field Theory—which made all sorts of unusual events possible.