The purpose of four-wheel drive is increasing traction and control in off-road or dangerous driving conditions. While a traditional two-wheel drive vehicle experiences a dangerous loss of stability on poor road conditions, a four-wheel drive experiences a much lower loss of control.
Most four-wheel drive vehicles are similar to two-wheel drive vehicles, with the exception that they feature a special part called a transfer case, which is located on the drivetrain after the transmission. Normally, torque generated by the engine travels through the gears of the transmission to the vehicle's two drive wheels. When the transfer case is activated, torque travels from this part equally to both of the vehicles axles. Most four-wheel drive vehicles allow turning the system on and off to increase fuel economy.
All-wheel drive is a distinct technology often confused with four-wheel drive. While four-wheel drive systems rely on the transfer case, all-wheel drive vehicles use a system of three differentials. These three differentials automatically supply the most power to whichever of the four wheels has the greatest amount of traction. While all-wheel drive increases handling performance, it does not provide the stability increase of four-wheel drive, especially at low speeds. Unlike four-wheel drive, which the driver can usually turn on or off at will, all-wheel drive systems are always on, which increases the vehicle's fuel consumption.