Intended only for use off-road or on extremely slippery surfaces, 4WD is a part-time system, meaning the driver must shift into and out of four-wheel drive by turning a knob, pushing a button, or yanking a lever.

Doing so locks the front and rear driveshafts together, keeping the front and rear axles turning at the same speed.

In sand, mud, and snow, this guarantees that, at a minimum, engine torque is always being sent to at least one front and one rear wheel, without relying on computers to predict or detect wheelslip. And that means there’s more power being put to the ground in slippery conditions to get you moving and keep you going.

But four-wheel drive is not designed to be used on paved roads and is reserved for vehicles with serious off-road capability, like the Jeep Wrangler or Jeep Gladiator.

Restrictions on Four-Wheel Drive Use

In a curve, a vehicle’s four wheels all rotate at different speeds. You can see this in the tracks left when a vehicle turns through fresh snow as each wheel traces a unique arc with a slightly different length. To allow the front and rear tires to spin at different speeds, all-wheel-drive systems use either a center differential or a clutch-pack coupling between the front and rear axles.

A four-wheel-drive system locks the front and rear driveshafts together, so that they rotate at the same speed and receive equal amounts of torque. Try driving in a tight circle on dry pavement with four-wheel drive engaged and you’ll both feel and hear an unnerving shudder as the driveline binds up. Sometimes the front tires will also hop and chirp.

This binding stresses the driveline and will cause the tires to wear unevenly if four-wheel drive is regularly used on paved roads. More important, using four-wheel drive on pavement can be dangerous, as the locked driveline can make turning difficult. On low-grip surfaces such as snow, ice, dirt, or mud, binding isn’t a problem because the tires can slide freely over the surface to reconcile the locked driveshafts with the need for each tire to trace a unique arc.

it’s possible to buy a vehicle with both an all-wheel-drive and a four-wheel-drive system. Optional in many modern full-size pickup trucks, these systems allow the driver to choose all-wheel drive with an Auto or 4Auto mode and four-wheel drive with the 4High setting. (These vehicles typically also include a rear-wheel-drive 2High mode and low-range 4Low mode.) A Land Rover Range Rover, for instance, behaves like an all-wheel-drive vehicle until you lock its center differential, at which point it becomes a four-wheel-drive truck.