When do you use the overdrive function?

All the time. I assume you are talking about an automatic transmission. In that case set it in it's highest position (overdrive) for all normal driving. This will give you the best gas mileage. Wrong. When you use the overdrive, he means when you take the overdrive off. It's used when you go down steep hills to slow the speed down. Or when you need extra power to overtake (pass) another car Both are correct. During the course of normal driving the car should be driven with O/D On. This will provide the best economy as well as improve ride comfort (engine isn't working as hard and is quieter). But when descending a hill it can be turned off to help control vehicle speed without using the brakes, or to assist the brakes in slowing the vehicle. This works much like 'down shifting' in a manual transmission vehicle. Also in most vehicles O/D should be turned off when towing or hauling large loads to keep the transmission from getting overworked. If a vehicle is working properly it will disengage O/D by itself when you use 'full throttle', such as when passing someone. There is no need to disengage it in this circumstance.