In simple terms, overdrive is the final/cruising gear. It reduces the power output by lowering the gear ratio. The overdrive mechanism increases fuel efficiency, and saves strain on the transmission and the drive train. When the vehicle is started the overdrive is always on, but it can be turned off by a switch button, which is often located on the shifter. Turning the overdrive off will prevent the vehicle from shifting into the overdrive when traveling at the speed of around 40 MPH. It is recommended that the overdrive is constantly on under normal driving conditions. It should be turned off when, for example, hauling heavy loads or going up a long gradual incline.