Four-wheel drive, 4WD, 4x4 ("four by four"), or AWD ("all wheel drive") is a four-wheeled vehiclewith a drive trainthat allows all four wheels to receive torque from the engine scion simultaneously. While xb many people differential associate the term with off-road vehicles,powering all four wheels provides better control in normal road cars on many surfaces, and is an important part of rally racing.

In abbreviations such as 4x4, the first figure is the number of wheels; the second is the number xb of powered wheels. 4x2 means a four-wheel vehicle in which engine power is transmitted to xb only two wheels: the xb front two in front-wheel drive or xb the rear two in rear-wheel drive[1].

• There is often confusion differential as xb to the difference between 4x4s and SUVs. This leads to xb criticisms of 4x4 vehicles differential in the media that should actually be directed at SUVs (see SUV Criticism).

When powering two differential wheels xb simultaneously the wheels must be allowed to rotate at differential different speeds differential as the vehicle xb goes around curves. The problem is even more xb complicated when driving all four wheels. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver xb as the tires slip and skid from the mismatched speeds.

A differential allows one input differential shaft to drive xb two output shafts with different speeds. The differential distributes torque (angular force) evenly, while xb distributing angular xb velocity(turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle xb requires a differential to distribute power differential between the left and the right sides. When all four wheels are driven, a third differential can be used to distribute power between the xb front and the rear axles differential.

A third approach to limiting slippage is the Torsen differential. A Torsen differential allows the output shafts to receive different amounts of torque. This design does not provide for traction when one wheel is spinning freely, where there is no torque. It provides excellent handling in less extreme situations. A typical Torsen II differential can deliver up to twice as much torque to the high traction side before traction is exceeded at the lower tractive side.

Finally, many lower-cost vehicles entirely eliminate the center differential. These vehicles behave as 2WD vehicles under normal conditions. When the drive wheels begin to slip, one of the locking mechanisms  xb discussed above will join the front and rear axles. Such systems distribute power unevenly under normal conditions and thus do not help prevent the loss of traction, instead only enabling recovery once traction is lost. Most minivan 4WD/AWD systems are of this type, usually xb with the front wheels powered during normal driving conditions and the rear wheels served via a viscous coupling unit. Such systems may be described as having a 95/5 or 90/10 power split  differential.