<P> In an era when different models of car with different wheel sizes could be accommodated by simply changing the final drive ratio, it made sense for all transmissions to use direct drive as the highest gear . As noted earlier, however, this would cause the engine to operate at too high an RPM for efficient cruising . Although adding the cruising gear to the main gearbox was possible, it was generally simpler to add a separate two - gear overdrive system to the existing gearbox . This not only meant that it could be tuned for different vehicles, but had the additional advantage that it could be offered as an easily installed option . </P> <P> With the use of front - wheel drive layouts, the gearbox and final drive are combined into a single transaxle . There is no longer a drive shaft between them and so the notion of "direct drive" isn't applicable . Although "overdrive" is still referred to, this is now mostly a marketing term to refer to any extra-high ratio for efficient cruising, whether it is achieved through the gearbox ratios, or by an unusually high final drive . </P> <P> Generally speaking, overdrive is the highest gear in the transmission . Overdrive allows the engine to operate at a lower RPM for a given road speed . This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway . When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70 + km / h (40 - 45 mph or more) depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears . Overdrive should usually be selected when the average speed is above 70 km / h (40 - 45 mph). </P> <P> The automatic transmission automatically shifts from OD to direct drive 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 next highest 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 . </P>

When do you use overdrive on a truck