When do you use overdrive on a car?

Overdrive is a part of the automatic transmission that helps the torque converter (the part of your transmission that converts power from the engine into power the transmission can use) cool itself. When the transmission shifts into overdrive, the torque converter goes into what's called 'lock-up', meaning that it stops turning so it can cool itself and the transmission no longer is using it. When pulling heavy loads, traveling on rough terrain, or up steep inclines, turn your overdrive off, otherwise you could cause damage to the transmission! Otherwise leave it on. I does help your gas mileage on long trips, but then, cruise control does too. Be sure and have the fluids in your transmission changed every 30K or once a year if you live in a highly populated area with lots of traffic, otherwise every 50K or two years.