Four-wheel drive (4WD) and all-wheel drive (AWD) are different types of automotive drive systems.
All-wheel drive systems continuously power all four wheels and provide maximum traction in all driving conditions. AWD is very useful on roads with slippery surfaces in and when driving in moderate off-road terrain. As AWD systems are full-time, there is no requirement for the driver to manually shift the transmission into AWD mode. AWD systems are lighter and more compact than 4WD systems and are increasingly being supplied with regular sedan or sports utility vehicles.
Four-wheel drive systems can be part-time or full-time. In a part-time system the driver manually shifts between two- and four-wheel drive, and a part-time 4WD vehicle should not be driven on dry roads when in 4WD mode.
Many 4WD systems are full-time. In a conventional full-time 4WD vehicle, a differential is used inside the transfer case to make 4WD use on pavement possible. This is done with a ring and pinion set or a planetary gear set. Each wheel receives 25% of the available torque, as long as the ground is level and traction is equal. This prevents wheel spin on normal roads but in off road conditions the wheels can lose traction and slip. To avoid this some full time 4WD vehicles provide a center differential lock as a means of spin control.
4WD vehicles have an additional set of low-range gears selected through a transfer case to assist in dealing with off-road conditions such as rocks, steep slopes, or deep water. This, and the fact that 4WD systems are typically heavier in construction than AWD, adversely affects the relative fuel economy of 4WD vehicles.