Four-wheel drive

Also called 4x4, 4WD, four by four, four wheel drive

4x4 means a vehicle with four wheels of which all four can be driven, the first number being the total wheels and the second how many receive engine power.

What does 4x4 mean?

It is a ratio, read as four by four. The first number is how many wheels the vehicle has. The second is how many of them the engine can drive. A 4x2 is an ordinary two-wheel-drive car or truck: four wheels, two driven. A 4x4 can drive all four. A six-wheeled military truck driving all six is a 6x6, and the same notation runs all the way up.

Which is a useful thing to know because it tells you what 4x4 does not mean. It does not mean all four wheels are being driven all the time, and on most vehicles it does not even mean all four are pulling when you have selected it.

Part-time, full-time and all-wheel drive

Traditional four-wheel drive is part-time. The vehicle drives its rear wheels normally and you engage the front axle through a transfer case when you need it. That system locks the front and rear driveshafts together, which is what makes it strong off-road and what makes it unusable on dry pavement, because the front and rear axles cover slightly different distances through a turn and something has to give.

Full-time four-wheel drive adds a differential in the middle so the axles can turn at different speeds, and can therefore be left engaged on tarmac. All-wheel drive usually means the same idea tuned for road traction and weather rather than terrain, typically without a low range.

Why one spinning wheel stops you anyway

Because an ordinary differential sends torque to whichever wheel needs least of it, which is the one with no grip. A part-time 4x4 with open differentials front and rear is really a two-wheel drive with one driven wheel per axle, and if one front wheel and one rear wheel are in the air you are going nowhere.

The fix is a locking differential, or a limited-slip, or a traction control system that brakes the spinning wheel. That is why serious off-road capability is a conversation about lockers rather than about the badge on the tailgate.