Simply speaking, drive gives you more power but also utilizes more gas. It is therefore prudent to be driving your car in overdrive when it is cruising at high speeds. This makes the engine run at lower rpm thus saving gas. If you have 5 gears in your car, overdrive is the 5th gear while all lower gears are called drive. Overdrive only needs to be turned off when you are slowing or when you are going uphill as this is when you require more power to push up the vehicle. The only drawback of overdrive is that it does not let the vehicle reach its maximum speed. If the maximum speed of your car is 115 mph, you can reach this speed with drive gears but you have to make a sacrifice with overdrive as it will not allow the car to go beyond a speed of 100mph. but as long as you are getting better mileage, who cares if the car is not attaining maximum speeds.While drive is 1:1 ratio (implying wheels are turning at exactly the same speed as the engine), overdrive is around 0.66:1 which allows you to go at higher speeds easily improving gas mileage. You thus achieve higher speeds with less noise of the engine with lesser consumption of gas with lower torque now available to you. This is bad news for acceleration or towing. So overdrive is just like having an extra gear in your car the lowers the revs of the engine with gas mileage going up. However, you must remember that overdrive is not for hauling up a steep slope or when pulling loads.Drive also gives more engine braking thanoverdrive to slow your vehicle ondowngrades.To save on gas, you must try to run the car in overdrive as much as possible. However, when going up a hill, use drive instead of overdrive as this is when you need more power from your car.Это из интернета, инструкцию неохота по-англ. перепечатывать, уж простите.