How does the overdrive work on a 2005 Vibe with an automatic transmission?

driving in town below 60km/hr put the overdrive off by pressing the O/D knob and O/D OFF will appear on the dash board, otherwise its always on by default.

overdrive works to reduce fuel consumption when driving at higher speed say over 80km/hr out of town or on a highway. Thus its not useful in short drive and stop distances such as in traffic situations.