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4WD and AWD systems explained
AWD is generally full-time four-wheel drive, e.g. Subaru, Jeep Grand Cherokee, Audi Quattro. All the wheels are providing power all the time, and can be used on any road surface.
4WD is basically a truck system, it locks differentials so all the wheels are spinning at the same rate i.e. Silverado, Ram, F-150. If you use this system on paved roads, you will damage your driveline.
There's all manner of "multi-wheel-drive" vehicles out there, all with their own unique system. The cheaper AWD vehicles actually only provide power for one side of the vehicle, while the more advanced systems actually do power all four wheels.