This paper describes an unique concept for the optimum design of tooth modifications of helical gears. The tooth modifications of helical gears will minimize transmission error under various loads with flexible supporting members, i.e. automobile transmissions. The key point of this concept is the amount of tooth modifications on each path of contact moved by misalignment under each respective target torque. Using this concept, it is possible to calculate the optimum 3-dimensional tooth modifications. The tooth modifications under light load will be a small curvature, and a large curvature under high load. Furthermore, through observation we can determine that the tooth surface modifications will have excellent impact on transmission error over a wide torque range. Finally, this method is verified experimentally in various misalignment conditions.