A drivetrain configuration where the vehicle typically operates with power delivered to the rear axle, but can engage a four-wheel drive system for enhanced traction when needed, combines the efficiency of two-wheel drive with the off-road capability of all-wheel drive. This setup is often selectable by the driver, allowing a shift between two-wheel drive and four-wheel drive modes depending on driving conditions. For example, a truck might use rear-wheel drive for highway driving, then switch to four-wheel drive to navigate muddy terrain.
This adaptable approach offers several advantages. Fuel economy is generally better in two-wheel drive mode compared to permanent all-wheel drive. Enhanced traction in challenging conditions like snow, mud, or sand improves safety and off-road performance. Historically, this selectable system emerged as a compromise between the on-road efficiency of rear-wheel drive vehicles and the need for enhanced traction in adverse conditions or off-road scenarios. This development played a key role in the evolution of versatile vehicles capable of handling both paved roads and rugged terrains.