Usage
Generally speaking, overdrive (OD) is the highest gear in the transmission. Most
automatic transmissions have three speeds and overdrive (fourth speed). Overdrive allows the engine to operate at a lower rpm for a given road speed. This allows the vehicle to realize better fuel efficiency, and often quieter operation on the highway. When it is switched on, the transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h [45+
mph] depending on the load). When it is off, the transmission shifting is limited to the lower gears. For normal driving conditions, operation of the overdrive should be enabled only if the average speed is above 70 km/h (approx. 43 MPH).
It may be necessary to switch it off if the vehicle is being operated in a mountainous area.
The automatic transmission automatically shifts from OD to third gear when more load is present. When less load is present, it shifts back to OD. Under certain conditions, for example driving uphill or towing a trailer, the transmission may "hunt" between OD and the third gear, shifting back and forth. In this case, switching it off can help the transmission to 'decide'. It may also be advantageous to switch it off if
engine braking is desired, for example when driving downhill. The vehicle's owner's manual will often contain information and suitable procedures regarding such situations, for each given vehicle.