<P> Although the designer was theoretically free to choose any ratio for the gearbox and final drive, there is one additional consideration which meant that the top gear of most gearboxes was 1: 1 or "direct drive". This is chosen for efficiency, as it does not require any gears to transmit power and so reduces the power lost by them . This was particularly important in the early days of cars, as their straight - cut gears were poorly finished, noisy and inefficient . The final drive then took this output and adjusted it in a fixed - ratio transmission arrangement that was much simpler to build . Final drive ratios of 4: 1 were common, meaning that the wheels would turn at one fourth the rate they would if directly connected to the engine . </P> <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>

When should overdrive be used on my truck