You have a 3 speed Auto with Overdrive making it a 4 speed. With O/D On, your car will engage O/D at speeds over 60km/h With O/D Off, your car stays in 3rd gear. It wont do any damage to your car unless it's always in redline, but you'll use alot more fuel. Most of the time you won't need it, as the Auto will kick down on it's own when you accelerate hard. Carlo's Tip for the day: Turn off O/D when braking, or going down steep hills. The car will pull up more quickly like selecting a lower gear in a manual.