Franchise agreements. Franchisors and franchisees are required to comply with the Franchising Code of Conduct. The Code applies to conduct occurring on or after 1 January 2015 in relation to all franchise agreements entered into, transferred, renewed or extended on or after 1 October 1998.
Generally speaking, overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway.
If you have an automatic transmission with both a D (Drive -- or sometimes it's labeled 3) and an OD (Overdrive), you can and should generally leave the gear selector in OD and let the transmission figure out what's best. Manual transmissions can have overdrive gears, too.