Recall your car only touches two things:  Air and the road.  Let me start with a practical example of air resistance.  Imagine you had a 4 X 4 sheet of plywood.  If you held it under your arm and started running, with the edge of the sheet facing into the wind; you could make some pretty good time.  Now, imagine if you held the plywood in front of you like a big window mirror, and started running with the entire 16 square feet facing into the wind.  You wouldn't go very fast. You would be experiencing the effects of aerodynamic drag.  What you're feeling is the air being pushed out of the way by the big sheet of wood.

Key Point #2.  Aerodynamic resistance is due to the vehicle having to push air out of the way, and around the car.

Facts about Air

Air is actually pretty thick, at least below 10,000 feet.  Above that altitude, air is thin, so there is a correspondingly lower amount of oxygen.  Cars dont have much power at high altitudes because their engines use less fuel to correspond with the lower availability of oxygen; this less fuel results in less power.  A rule of thumb says that for every 1000 feet above sea level, your car loses 4-5% of its power.  Of course the thin air is still dense enough to cause aerodynamic resistance.

Another Example

Heres another example of aerodynamic resistance.  Take a walk and while walking, hold your arms up.  Face your hands into the wind, and then point your fingers into the wind.  Feel anything?  No.  Now, and I know youve all done this as a kid.  Now, go out in your car, with someone else driving.  Unroll the passenger window and stick your arm outside.  Hold your hand flat, hand opened up, then point it into the wind like a spear.  Notice that even at speeds around 30 mph, the shape that your hand presents to the airflow makes a dramatic difference to how hard the air pushes back.

What you did was make your hand alternate between un-aerodynamic and aerodynamic.  And further, you noticed that speed amplified this effect.  While walking, it didnt matter, but get up some speed, and whoa buddy, the air felt substantial.  This is critical to understanding aerodynamic resistance.  Pushing an object through the air gets harder the faster you go.

Also, drag goes up the larger the object is and the less streamlined it is.   So, the worst object to try and push through the air would be a big square object like a delivery truck, and the best would be a slick sports car, like a Lotus, Ferrari, or Corvette. 

The factors in aerodynamic resistance or drag are:

- The length and shape of the car (calculated into the vehicles Drag coefficient)

- The frontal area of the car exposed to the air

The frontal area, the second factor, is easy to grasp.  Make yourself small on your bike and you go faster.  Make yourself big by sitting up and holding out your arms and youll go slower.  Stand in front of a car and measure how wide it is and how tall it is, and multiply the two.  Then, subtract away the open area underneath the car, between the tires.  This is the frontal area.

Common drag coefficients (Cd) are:

- Motorcycle, 0.50

- Delivery truck, 0.60-0.80

- 1970s American sedan, 0.45

- Modern sports car, 0.30

You can see based on the Cd that a sports car is twice as aerodynamic as a delivery van.  In another way of looking at it, the sports car is half as dirty as the van. 

Now consider that drag is the product of drag coefficient and frontal area multiplied together.  Because the frontal area of every sports cars is between 15-23 square feet, and the frontal area of a delivery truck is about 45-65 square feet, the sports car clearly has much less drag.   In fact, the drag is about 1/5, when you consider the smaller frontal area combined with the smaller coefficient of drag.  Can you go 5 times faster?  No, sorry.  It doesnt quite work that way.

Lets take this further.   In addition we know that aerodynamic drag increases greatly with speed, in fact, it does so by the square of the speed.  That is an important principle with aerodynamic resistance.

The Government Fought Drag to Increase Mileage

This is EXACTLY why the US Government created the federal 55 mph speed limit in the 1970s.  They wanted to slow us down and save fuel.  They felt that going 55 was a good compromise between getting somewhere quickly, safety and economy.  Today, cars are likely to get nearly as good mileage at 65 mph as they do at 55 mph because they are more aerodynamic and have engines that are relatively efficient over a greater range of speeds.  Sadly, most are not great at any one speed, just decent at a wider range of speeds.  Dont forget this point.

Key Point #3:  Modern cars are more aerodynamic than ever before, and their engines are decently fuel efficient over a wider range of speeds, however few of them are extremely efficient at any specific speed.

Ill give you an example.  The first car is a 1974 Oldsmobile Cutlass Supreme, a 4,300-lb car with a drag coefficient of about 0.45.  It had a 5.7L V8 engine, and a 3-speed automatic transmission.  This car got 22 mpg consistently on the highway.  The second car was a 1990 Lexus LS400.  It is a 3,900-lb car with a drag coefficient of 0.29.  It had a smaller, 4.0L V8 and a 4-speed automatic transmission.  The smaller engine and lighter weight should give car #2 better city mileage, which it did achieve.  The fourth gear and astoundingly good drag coefficient should have translated into a highway rating much higher than the Oldsmobile.  Curiously, it got 23 mpg on the highway, just 1 mpg better.  This is just one example of how modern makers have failed to capitalize on technological improvements to save fuel. 

Back to aerodynamics.  To have a higher top speed or to prevent excessive fuel consumption at higher vehicle speeds, it is important it is to be slick or clean, aerodynamically.  What does it mean to be slick or have a good, low aerodynamic drag coefficient?  Well, let me explain what goes into your cars drag coefficient in the first place.  The drag coefficient is a single number that represents the entire shape of your car, mathematically.

Elements of the Drag Coefficient:

- Length of the car

- Shape of the car (sleek, pointed, or boxy and square, tapered or square rear)

- Number and size of protrusions (underneath, on top, on the sides).  Side mounted mirrors are protrusions that increase drag

- Smoothness of the car (rounded fenders, smooth transitions from the windows to the roof; also described as lack of sharp edges and sharp angles)

Remember when the Ford Taurus debuted in the mid-1980s?  It had a very low Cd for the time, about 0.33.  That was a big improvement in aerodynamics for mass produced passenger cars.  At that time, the average car had an aerodynamic drag coefficient of about 0.40 to 0.45.  The reduction to 0.33 represented a potential to improve highway fuel mileage by as much as 25%.  We know from above that city mileage was not likely to be better because at speeds up to about 35 mph, aerodynamics are still secondary, having minimal effects on mileage.  In the city, the stopping and going (accelerating uses the most fuel) and idling in traffic define the efficiency or lack of.

To make a drag coefficient of 0.27 (Cd has no units) which is the state-of-the-art for sedans and sports cars in 2003, automakers now design cars with flush door handles, long and sleek side mirrors, plastic underbody fairings to make the air flow smooth underneath, exhaust pipes flush with the underside of the car, rounded fender and roof edges and corners, and tapered rear ends. 

Here are a few examples of drag coefficients, with comments:

- 1972 Corvette, Cd = 0.50  Swoopy shapes plus lots of sharp edges

- 1984 Corvette, Cd = 0.34  Totally new car, smooth and straight lines

- 1986 Taurus, Cd = 0.33  First mass produced car that took Cd seriously

- 1990 Lexus LS400 Cd = 0.30  Smooth, tapered, rounded, underbody fairings

- 1999 Corvette, Cd = 0.29  Smooth and straight underneath

Formula for Aerodynamic Drag:  To get the overall force of aerodynamic drag on your car, use this formula:

·         Drag = Cd * ½ * p * V2 * A

Cd is the coefficient of drag, p is the density of the air (in lbs-mass per cubic foot), V2 is the speed of the car, squared, and A is the frontal area (roughly equal to the height of the car times the width of the car).

Lets do an example:

Car:  1972 Corvette at 55 mph (80.7 ft per sec).  Cd = 0.50, p = density of air (.00237 lbm/ft3), A = 18.5 sq ft.  Lets calculate the drag force of the air on the car at 55mph.

Drag = .5*.5*.00237*80.7*80.7*18.5 = 71.38 pounds

Lets repeat the example at 110 mph (161.4 ft per second).  Although the speed doubled, the aerodynamic drag will go up by a factor of four (4 times higher) because of the V-squared variable in the above equation.

Drag = .5*.5*.00237*161.4*161.4*18.5 = 285.51 pounds

Finally, lets repeat again with a 1984 Corvette at 110mph.  This is a much more aerodynamic car, but the car has slightly more frontal area.

Drag = .34*.5*.00237*161.4*161.4*21 = 220 pounds

So, at 110 mph, with 220 lbs of resistance vs 285 lbs, the aerodynamic car has 23 % less air resistance than the old one, which will directly relate to better fuel economy. 

The lower drag value means the newer car needs less power to reach 110 mph than the older one.  It will use less gas, and be able to go faster; it will also run cooler because 95% of engine heat is due to burning gas (so if you burn less gas, the engine will generate less heat).   Now you can see the advantage of designing cars with the aid of a wind tunnel.  Of course, the rolling resistance of the car was not considered here, and is also slightly lower for the 1984 car because of lighter weight and a better driveline design.