What's The Difference Between All-Wheel Drive and 4-Wheel Drive?

With winter catching up to us it is a good time to review the differences between four-wheel drive (4WD) and all-wheel drive (AWD) vehicles.

 
Let's start with four-wheel drive vehicles.  4WD systems have been around the longest and have been tried and improved over that time.  4WD is when the vehicle is set up to split the power between all four wheels equally.  The good thing about it is you have either a button or a lever in the vehicle that allows you to shut it off when not needed and save on gas.  The bad thing is when engaged all four wheels need to spin at the same speed which is not good for turning since the outer wheel must spin faster than the inner wheel.  If left this way for too long, it can cause damage to the vehicle.  Nowadays you will mostly find 4WD systems on trucks and large SUVs, and they are much more refined with many having an Auto 4WD mode where the vehicle will sense slippage and lock in the 4WD and then lock it out when not needed.  Many also have a 4WD high mode and a 4WD low mode.  The 4WD high mode allows you to run at higher speeds when engaged (usually around 55 mph but check your owner's manual to make sure) and higher tolerances for the wheels spinning at the same speed.  4WD low mode performs more like traditional 4WD systems, in that it should only be used at low speeds and when you're really stuck. 
4WD pros:
-the best traction in off-road conditions
-can be turned off to improve fuel economy
4WD cons:
-adds weight and complexity to vehicles
-can't be used in all weather conditions
 
Now we are seeing a lot more all-wheel-drive vehicles on the market.  Many small SUVs, some cars, and even a few larger SUVs and trucks are now coming out with AWD.  The biggest difference between 4WD and AWD drive is that AWD is on all the time.  AWD systems are set up so that the vehicle transfers power away from wheels that are slipping and to the wheels that are gripping. The good thing is the computer on the vehicle can sense the changes in driving conditions many times faster than we can and adjust accordingly.  As mentioned earlier the system can not be shut off and that is why AWD vehicles have lower gas mileage ratings.  AWD systems are more complex, have more parts, and therefore can have more issues over the life of the vehicle.  
AWD pros:
-provides increased traction under all road conditions
-works all the time
AWD cons:
-reduces fuel economy
-increases weight and complexity of the vehicle
 
The best way to decide which is best for you is to think of the winter conditions in your area and how extreme they can become.  The more extreme, the more you may need to consider a 4WD vehicle.  Also, if you're sticking to the roads AWD should do well for you, but the more off-road driving you do, the more you may want to consider a 4WD.

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