All-Wheel Drive vs Four-Wheel Drive vs Two-Wheel Drive

10 March 2014; Over the Internet --

The Number One Son and Grumpy have been having a vigorous discussion regarding the characteristics, advantages and disadvantages of the subject variations of automobiles (and trucks and such). If you are expecting a conclusion at the end of this POST, then stop here; you will find none. This is merely Grumbling Grumpy's summary of our discussions.

And since this is MY BLOG, it will necessarily be biased toward MY opinions.

Some definitions (and abbreviations): 

1. All-Wheel Drive (AWD)-- a configuration in which all four wheels are driven through a transfer case which includes a differential gear so that all wheels may rotate at the same speed when the vehicle is going strait ahead but allow the front wheels to rotate at a suitably higher speed when the vehicle is turning a corner. Examples include the Subaru.
2. Four-Wheel Drive (ODFWD)-- a configuration in which the front wheels (usually) may be engaged through a transfer case, either automatically by a computer or manually by the driver. Such configuration may not be operated on dry or wet pavement, since the gear ratio of the front wheel drive is slightly "faster" than that for the rear wheels, thus creating a stress in the drive system when the front wheels are not able to spin slightly (or the rear wheels not able to slide) when the vehicle is going straight ahead.
3. Two-wheel drive (2WD)-- a configuration in which only one set of wheels (either front or rear) are driven. This is the conventional drive configuration for most passenger vehicles and trucks.

 There is definitely a difference in fuel mileage (MPG) between the 2WD configuration and the other two. This MPG difference is readily understood if one considers the extra weight and friction (due to extra rotating mechanical elements) in the configurations having the capability for four-wheel drive. Approximately 4-5 MPG difference was generally agreed upon by the two of us.

Grumpy had the opportunity to evaluate the MPG of a Subaru Outback last year. I was pleased and in fact surprised that the average MPG for the most of the cross-country trip was about 25 MPG; my 2004 Accord would have attained about 32 MPG.  This respectable MPG was despite the "boxy" shape of the Outback and the fact that we had an "Aerodynamic*" luggage carrier on top!

An Environmental Protection Agency (EPA) published MPG difference between the AWD and ODFWD was not as easily understood or explainable, however. The AWD configuration necessitates a differential in the transfer case and thus would have a slight weight penalty over the ODFWD. However, this would not appear to explain a measurable MPG difference. The EPA numbers suggest differently.

However, reference to the "Truth About EPA ... Estimates" reveals that only about 15% of new vehicles are actually tested each year. The EPA depends on the manufacturers claims for the rest! Thus, I believe that the EPA numbers do not have the veracity that some people attribute to them. They are especially suspect when comparing the EPA numbers for similar automobiles by different manufacturers; vehicles from one or none may be from actual EPA tests and the others all from manufacturers' "claims".

As I said at the beginning, no conclusions are offered; only "caveat emptor" -- that the buyer should beware! Comments to this dissertation are invited.

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* "Aerodynamic" as used here is a gross exaggeration and a large bit of sarcasm! However, that is the appellation given by its manufacturer. The luggage carrier was a flexible plastic, water-tight bag of approximately 3' x 4' x one foot high; it looked like a couple partially-filled 33-gallon garbage bags thrown cross-wise on top of this otherwise fairly tidy station-wagon body. I estimate that it added about 3-4 square feet of additional frontal area and considerable "skin drag" as it had numerous wrinkles along all sides.