All-wheel drive vehicle

The most common forms of all-wheel drive are:

1x1

All unicycles
Reflects one axle with one wheel capable of being powered.

2x2

Some motorcycles and bikes
Reflects two axles with one wheel on each capable of being powered.

4×4 (also, four-wheel drive or 4WD)

Reflects two axles with both wheels on each capable of being powered.

6×6 (also, six-wheel drive or 6WD)

Reflects three axles with both wheels on each capable of being powered.

8×8 (also, eight-wheel drive or 8WD)

Reflects four axles with both wheels on each capable of being powered.

Vehicles may be either part-time all-wheel drive or full-time:

On-demand or Part-time

One axle is permanently connected to the drive, the other is connected as needed

Full-time or Permanent

All axles are permanently connected, with or without a differential.

Independent

The wheels are driven, but not dependent on a central mechanical power coupling.

In the context of hybrid and electric vehicles, the North American designation of 'all-wheel-drive' may differ. In some hybrid vehicles, the combustion engine is only mechanically connected to the front wheels, while the rear wheels are powered independently by an electric motor; this may be marketed as eAWD. Furthermore, many electric vehicles have individual, unconnected motors powering each axle, or even each individual wheel, also without any center differential.

When tire grip is good during road driving, a differential is used between the axles to avoid driveline windup. This is not required off-road, as the limited grip allows the tires to slip. All-wheel drive vehicles designed for extensive off-road use may not have such a differential, and so they suffer from wind-up when used on-road. Selectable 4WD also avoids this problem and requires only a simple dog clutch in the transfer case, rather than a differential. For this reason, most early off-road vehicles used that system; e.g., Jeep, Land Rover.

As vehicles became more sophisticated and tires gave better winter performance in the 1960s, there was an interest in giving the benefits of all-wheel drive to conventional cars: not for off-road use but for winter use in snow and ice or on wet roads. Exotic vehicles such as the high-powered Jensen FF followed by the AMC Eagle, Subaru Leone and Audi Quattro series were the first to offer all-wheel drive in a high-speed road-based car. These, particularly the Quattro, would extensively develop this drivetrain with the use of viscous couplings and differentials to provide a safe and drivable car. The first off-road / on-road hybrids such as the Range Rover also chose the permanent all-wheel-drive system rather than manual selection.