Four-wheel-drive (4WD) and all-wheel-drive (AWD) systems can dramatically increase a vehicle's traction and handling ability in rain, snow, and off-road driving. Considering that the vehicle's only contact with the road is the small area of the trees, driving and handling is vastly improved if the work load is spread out evenly among four wheels rather than two.

Factors such as the side forces created by cornering and wind gusts have less effect on vehicles with four driving wheels. The increased traction also makes it possible to apply greater amounts of energy through the drive system. Vehicles with 4WD and AWD can maintain control while transmitting levels of power that would cause two wheels to spin either on take off or while rounding a curve. The improved traction of 4WD and AWD systems allows the use of tires narrower than those used on similar 2WD vehicles. These narrow tires are less expensive. They also tend to cut through snow and water rather than hydroplane over it.

Four-wheel-drive vehicles can transfer large amounts of horsepower through the drivelines without fear of drivetrain damage. This is not necessarily true of front-wheel-drive vehicles and all-wheel-drive vehicles (both of which use transaxles).

Both 4WD and AWD systems add initial cost and weight. With most passenger cars, the weight problem is minor. A typical 4WD system adds approximately 170 pounds to a passenger car. An AWD system adds even less weight. The additional weight in larger 4WD trucks can be as much as 400 pounds or more.

The systems also add initial cost to the vehicle. Vehicles equipped with 4WD and AWD require special service and maintenance not performed on 2WD vehicles. However, the slight disadvantages of 4WD and AWD are heavily outweighed by the traction and performance these systems offer. Their popularity is increasing at a rapid rate, and technicians must be prepared to diagnose and repair these systems.

4WD versus AWD

Due to the many names manufacturers give their drive systems, it is often difficult to clearly define the difference between 4WD and AWD.

In this text, 4WD systems are those having a separate transfer case. They also give the driver the choice of operating in either 2WD or 4WD through the use of a shift lever or shift button. The locking hubs used in 4WD systems can be either manual or automatic locking. Finally, a 4WD system may or may not use an interaxle differential or viscous clutch to distribute driving power to its front and rear drivelines.