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AWD is a type of 4WD sometimes called "full-time 4WD." It may not be driving all 4 wheels all the time, but if it isn't, it will automatically detect when extra traction is needed and shift power according to that need. There are many, MANY different variations on how this is done.


AWD will ALWAYS have 3 differentials. Two between the front and rear wheels to vary power left to right, and one between the front and rear differentials to vary power front to back.

4WD MAY have 3 diffs, but it could lack the center differential.



AWD will usually not use locking differentials for all 3 differentials, and may not use locking differentials at all.

4WD will very often have all differentials capable of locking.
Locking dif - Wikipedia, the free encyclopedia... The characteristics of a locking diff are advantageous in situations where traction between wheels varies wildly (like one wheel is off the ground and has zero traction while the other is on rocks and has a lot of it), which mostly occur off-road. That is why off-road vehicles like Jeeps and stuff are almost always referred to as 4WD, while things like Subarus, which are designed more for handling difficult weather and unmaintained roads, are AWD.

Many off-road 4WD systems are not optimized for road use. They don't allow the wheel speeds to be different from left to right when the car is turning, for example (the outside wheel has to travel farther, and thus must turn faster). This leads to uneven tire wear, poor fuel economy, and can damage the differentials. Thus, most 4WD vehicles have a lever or switch to disable two of the drive wheels (usually the front, except for some old 4WD cars, like old Subaru GLs and Loyales). AWD does not need this, because either the system will be switched to 2WD in most driving conditions, or the system is designed to cope with spending 100 percent of the time driving all 4 wheels (and regardless of how they're putting power down, they can all vary the speed left to right)


There is a lot of overlap between AWD and 4WD, though...some 4WD systems in trucks would be called AWD in a car. The only consistent, 100 percent of the time difference is that AWD is not controlled by the driver. If it can be shut off, it is done automatically.


edit: Just noticed the category. For Subaru specifically, the practical difference is that with a 4WD, you CANNOT drive around like that all the time. It is NOT intended for highway use, and you really shouldn't go more than about 30 or 40 mph with 4WD turned on. The AWD systems aren't all the same in how they work, but as far as the driver is concerned, they work automatically. You never have to press any buttons or anything to activate it, it does what it needs to do at all times without your input. The only exception to this is in the STI, which has a dial in the center console that allows you to control how much of the torque goes to the front wheels and how much goes to the back (by controlling the center differential). IIRC, it can go up to 90 percent front or 90 percent rear.