4WD versus AWD

The term four-wheel drive describes truck-like vehicles that may allow the driver to manually switch (sometimes with an automatic option) between two-wheel drive mode (if available) for streets and four-wheel drive mode for low traction conditions such as ice, mud, snow, slippery surfaces, or loose gravel.

All-wheel drive (AWD) is often used to describe a "full time" 4WD that may be used on dry pavement without damaging the differentials, although the term may be abused when marketing a vehicle. AWD can be used on dry pavement because it employs a center differential, which allows each axle to rotate at a different speed. This eliminates driveline binding, wheel hop, and other driveline issues associated with the use of 4WD on dry pavement. For vehicles with more than four wheels, AWD means all wheels drive the vehicle, to varying degrees of engagement, while 4WD means only four of the wheels drive the vehicle continuously. For example, an AWD vehicle with six wheels is often described as a 6x6, the M35 2-1/2 ton cargo truck being one of the best-known examples (dual wheels on the rear axles are not counted as additional drive wheels).

Because all 4 tires in a full time AWD system are connected by a system of differentials, they are potentially very susceptible to torque reduction when a wheel loses traction. Without sophisticated traction control systems, they would become immobilized when any one of the four tires lost traction. A traditional part time 4WD system does not connect the front and rear via a differential, and therefore does not suffer any front/rear torque reduction - if a front tire loses traction, it does not reduce torque delivered to the rear tires, even without traction control systems.