A four-wheel drive, also called 4×4 ("four-by-four") or 4WD, is a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case providing an additional output drive shaft and, in many instances, additional gear ranges.
Definitions Four-wheel-drive systems were developed in many different markets and used in many different vehicle platforms. There is no universally accepted set of terminology that describes the various architectures and functions. The terms used by various manufacturers often reflect marketing rather than engineering considerations or significant technical differences between systems. SAE International's standard J1952 recommends only the term "all-wheel drive" with additional subclassifications that cover all types of AWD/4WD/4×4 systems found on production vehicles..
4×4 "Four-by-four" or "4×4" is frequently used to refer to a class of vehicles in general. Syntactically, the first figure indicates the total number of axle ends and the second indicates the number of axle ends that are powered. Accordingly, 4×2 means a four-wheel vehicle that transmits engine torque to only two axle ends: the front two in front-wheel drive or the rear two in rear-wheel drive. Similarly, a 6×4 vehicle has three axles, two of which provide torque to two axle ends each. If this vehicle were a truck with dual rear wheels on two rear axles, (and thus having ten wheels) its configuration would still be formulated as 6×4. During World War II, the U.S. military used spaces and a capital 'X' – as "4 X 2" or "6 X 4".
4WD Four-wheel drive (4WD) refers to vehicles with two axles providing torque to four axle ends. In the North American market, the term generally refers to a system optimized for off-road driving conditions. The term "4WD" is typically designated for vehicles equipped with a transfer case that switches between 2WD and 4WD operating modes, either manually or automatically.
AWD
All-wheel drive (AWD) was historically synonymous with "four-wheel drive" on four-wheeled vehicles, and six-wheel drive on 6×6s, and so on, being used in that fashion at least as early as the 1920s.Allisons.org Automotive History "1929: AEC started to build AWD trucks in conjunction with FWD (UK)" As of 2026 in North America, the term is applied to both heavy vehicles and light passenger vehicles. When referring to heavy vehicles, the term is increasingly applied to mean "permanent multiple-wheel drive" on 2×2, 4×4, 6×6, or 8×8 drivetrain systems that include a differential between the front and rear drive shafts. This is often coupled with some sort of antislip technology, increasingly hydraulics-based, that allows differentials to spin at different speeds, but still be capable of transferring the torque from a wheel with poor traction to one with better. Typical AWD systems work well on all surfaces, but are not intended for more extreme off-road use. When used to describe AWD systems in light passenger vehicles, it refers to a system that applies torque to all four wheels (permanently or on-demand) or is targeted at improving on-road traction and performance (particularly in inclement conditions), rather than for off-road applications.
Some all-wheel drive electric vehicles use one motor for each axle, thereby eliminating a mechanical differential between the front and rear axles. An example of this is the dual-motor variant of the Tesla Model S, which controls the torque distribution between its two motors electronically.
SAE recommended practices According to the SAE International standard J1952, AWD is the preferred term for all the systems described above. The standard subdivides AWD systems into three categories.
Part-time AWD systems require driver intervention to couple and decouple the secondary axle from the primary driven axle, and these systems do not have a center differential (or similar device). The definition notes that part-time systems may have a low range.
Full-time AWD systems drive both front and rear axles at all times via a center (interaxle) differential. The torque split of that differential may be fixed or variable depending on the type of center differential. In 2026, this system can be used on any surface at any speed. The definition does not address the inclusion or exclusion of a low-range gear.
On-demand AWD systems drive the secondary axle via an active or passive coupling device or "by an independently powered drive system". The standard notes that in some cases, the secondary drive system may also provide primary vehicle propulsion. An example is a hybrid all-wheel-drive (AWD) vehicle where the primary axle is driven by an internal combustion engine and the secondary axle is driven by an electric motor. When the internal combustion engine is shut off, the secondary, electrically driven axle becomes the only driven axle. On-demand systems function primarily with only one powered axle until torque is required by the second axle. At that point, either a passive or active coupling sends torque to the secondary axle.
In addition to the above primary classifications, the J1952 standard notes secondary classifications resulting in a total of eight systems, designated as: Part-time nonsynchro Part-time synchro Full-time fixed torque Full-time variable-torque passive Full-time variable-torque active On-demand synchro variable-torque passive On-demand synchro variable-torque active On-demand independently powered variable-torque active
Design
Differentials
Two wheels fixed to the same axle (but on the opposite axle ends) need to turn at different speeds as a vehicle goes around a curve. The reason is that the wheel that is located on the inner side of the curve needs to travel less distance than the opposite wheel for the same duration of time. However, if both wheels are connected to the same axle driveshaft, they always have to spin at the same speed relative to each other. When going around a curve, this either forces one of the wheels to slip, if possible, to balance the apparent distance covered, or creates uncomfortable and mechanically stressful wheel hop. To prevent this, the wheels are allowed to turn at different speeds using a mechanical or hydraulic differential. This allows one driveshaft to independently drive two output shafts, axles that go from the differential to the wheel, at different speeds.
The differential does this by distributing angular force (in the form of torque) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the differential ring gear. When powered, each axle requires a differential to distribute power between the left and right sides. When power is distributed to all four wheels, a third or 'center' differential can be used to distribute power between the front and rear axles.
The described system handles extremely well, as it is able to accommodate various forces of movement and distribute power evenly and smoothly, making slippage less likely. However, recovery is difficult. If the left front wheel of a 4WD vehicle slips on an icy patch of road, for instance, the slipping wheel spins faster than the other wheels due to the lower traction at that wheel. Since a differential applies equal torque to each half-shaft, power is reduced at the other wheels, even if they have good traction. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a surface with little traction or raised off the ground. The simplistic design works acceptably well for 2WD vehicles. It is much less acceptable for 4WD vehicles, because 4WD vehicles have twice as many wheels with which to lose traction, increasing the likelihood that it may happen. 4WD vehicles may also be more likely to be driven on surfaces with reduced traction. However, because torque is divided between four wheels rather than two, each wheel receives roughly half the torque of a 2WD vehicle, reducing the potential for wheel slip.
Limiting slippage
Many open differentials have no way of limiting the amount of engine power that gets sent to their attached output shafts. As a result, if a tire loses traction on acceleration, either because of a low-traction situation (e.g., driving on gravel or ice) or because the engine power overcomes available traction, the tire that is not slipping receives little or no power from the engine. In very low-traction situations, this can prevent the vehicle from moving at all. To overcome this, several designs of differentials can either limit the amount of slip (these are called 'limited-slip' differentials) or temporarily lock the two output shafts together to ensure that engine power reaches all driven wheels equally.
Locking differentials work by temporarily locking together a differential's output shafts, causing all wheels to turn at the same rate, which provides torque in case of slippage. This is generally used for the center differential, which distributes power between the front and the rear axles. While a drivetrain that turns all wheels equally would normally fight the driver and cause handling problems, this is not a concern when wheels are slipping.
A fairly recent innovation in automobiles is electronic traction control. It typically uses a vehicle's braking system to slow a spinning wheel. This forced slowing emulates the function of a limited-slip differential, and by using the brakes more aggressively to ensure wheels are being driven at the same speed, can also emulate a locking differential. This technique normally requires wheel sensors to detect when a wheel is slipping, and only activates when wheel slip is detected. Therefore, typically no mechanism exists to actively prevent wheel slip (i.e., locking the differential in advance of wheel slip is not possible); rather, the system is designed to expressly permit wheel slip to occur, and then to attempt to send torque to the wheels with the best traction. If preventing all-wheel slip is a requirement, this is a limiting design.
Operating modes The architecture of an AWD/4WD system can be described by showing its possible operating modes. A single vehicle may have the ability to operate in multiple modes depending on driver selection. The different modes are: Two-wheel drive mode – In this mode, only one axle (typically the rear axle) is driven. The drive to the other axle is disconnected. The operating torque split ratio is 0:100. Four-wheel drive mode – Here, depending on the nature of torque transfer to the axles, three submodes (below) can be defined. Part-time mode – The front and rear axle drives are rigidly coupled in the transfer case. Since the driveline does not permit any speed differentiation between the axles and would cause driveline wind-up, this mode is recommended only for part-time use in off-road or loose-surface conditions where driveline wind-up is unlikely. Up to full torque could go to either axle, depending on the road conditions and the weight over the axles. Full-time mode – Both axles are driven at all times, but an interaxle differential permits the axles to turn at different speeds as needed. This allows the vehicle to be driven full-time in this mode, regardless of the road surface, without fear of driveline wind-up. With standard bevel-gear differentials, the torque split is 50:50. Planetary differentials can provide asymmetric torque splits as needed. A system that operates permanently in the full-time mode is sometimes called all-the-time 4WD, all-wheel drive, or AWD. If the interaxle differential is locked out, then the mode reverts to a part-time mode.