Bro... let me first explain the meaning of word "Overdrive" to you. When moving in lower gears, your car is low on speed n high on power and as u shift the gears up, the inverse relationship starts to act. Your car gains speed but the power gradually decreases and the same is visible on your RPM meter. In a layman's language, In lower gears, the engine revolution will be more than the revolution of your wheels. Input revolution will be more than output revolution. As u shift the gears up, the engine revolution starts to drop and your wheels revolution increases. At one point, the engine revolution will become equal to the revolution of your wheels. input=output. That's known as a 1:1 drive or a direct drive. The moment your wheels start revolving faster than the engine revolution, u achieve an Overdrive. input<output. Generally, 5th gear in almost all vehicles is an Overdrive gear. Within city, keep it off... u don't need speeds above 70-80 kmph... going uphill or downhill, keep it off because u need power n engine braking, not speed. when u need a quick overtake, turn it off. Your car needs power now. Top gear of your car will not be engaged when it's off. On highways keep it on. It will give u fuel economy on high speed. Within city, on lower speeds, it hardly matters whether it's on or off .. You will otherwise be limited to 3rd gear.