by Garet Harris

Created on: April 12, 2009

The modern sport utilities vehicle or SUV has become a beloved icon of American family culture. While providing extreme levels of safety and comfort for it's occupants, it has a down side when it comes to fuel economy. The why is actually a very simple explanation, It is based upon several factors, these factors are; weight, wind sheer coefficient, and draft. I know these terms sound strange when being related to a vehicle, but there affects dictate your fuel mileage. Let's take a look at the terms and what they actually mean for the SUV owner.

First up is weight, now this is really a no brainer, obviously if you have two vehicles driving at the same speed, but one vehicle weighs twice as much as the other ,the heavier of the two vehicles will require more fuel to maintain the same velocity. It is basic physics on this one, it requires horsepower and torque to move a rolling mass at a given velocity. This horsepower generation requires fuel, the more weight, the more power necessary, the more fuel required to make power. This is part of why pulling a trailer uses more fuel per mile.

Now lets look at wind sheer coefficients, while I'm not going to get really technical about this I will merely state that wind sheer is a by product of the vehicle moving through the air. The more surface area presented to push air, the higher the wind shear coefficient. Basically an SUV with a fairly squared front end plus the wind shield plus the ride height is going to push much more air than a car. The car has an extremely low profile and is much more aero dynamic. To give an example Take a piece of card board and a wooden spoon. Push the cardboard flat through the air as fast as you can, now try the spoon. The spoon was much faster wasn't it. It is precisely the same principle with vehicles.

Now lets talk draft, if you watch NASCAR at all, you have probably heard this term bantered around. We are going to be applying the term differently than what they discuss. Draft on a vehicle is the is the hole or space left vacant of air molecules right upon leaving that exact spot. Think of a wave, that empty trough behind the wave is the vacant spot, It is the same with a vehicle moving through the air a t high speed. Now this hole causes a vacuum that actually creates drag on the vehicle requiring the vehicle to create more power to make up for that drag. The larger the hole left the more drag or draft applied to a vehicle. Have you ever seen a motor cycle riding about two feet of off a semi trailer's tail. He is getting a free ride, the draft or vacuum created by such a large package moving through the air is more than enough to pull a medium size bike and rider along in it's wake.

When you combine all of these factors together you get a scientific profile of how that vehicle moves through atmosphere. It is much the same with aeronautic design albeit much more critical, but none the less these principles apply. This is why many of the manufacturers started wind tunnel testing in the 1980's.

It was the only means of truly predicting fuel mileage numbers, as well as getting a better understanding of how aerodynamics affect profile when confronted with atmospheric resistance. This is why your 7500 lb SUV gets 17mpg and your buddy's 2500 lb car gets 32 mpg. Happy motoring.

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