Basics

Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a sub field of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used to correspond with gas dynamics, with the difference being that gas dynamics applies to all gases. Understanding the motion of air around an object enables the calculation of forces and moments acting on the object. Typical properties calculated for the motion of air includes velocity, pressure, density and temperature as a function of position and time. By defining a control volume around themotion of air, equations for the conservation of mass, momentum, and energy can be defined and used to solve for the properties. The use of aerodynamics through mathematical analysis, practical approximations, wind tunnel experimentation, and computer simulations form the scientific basis for heavier-than-air flight. Aerodynamic problems can be identified in a number of ways.
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Types of aero

External aerodynamics is the study of flow around solid objects of many shapes. Evaluating the lift and drag on an airplane or the shock waves that form in front of the nose of a rocket are examples of external aerodynamics. Internal aerodynamics is the study of flow through passages in solid objects. For instance, internal aerodynamics encompasses the study of the airflow through a jet engine or through an air conditioning pipe.
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Speed

The ratio of the problem's characteristic flow speed to the speed of sound comprises a second arrangement of aerodynamic problems. A problem is called subsonic if all the speeds in the problem are less than the speed of sound, transonic if speeds both below and above the speed of sound are present, supersonic when the characteristic flow speed is greater than the speed of sound, and fast when the flow speed is much greater than the speed of sound. Aerodynamicists disagree over the precise definition of fast flow; minimum Mach numbers for fast flow range from 3 to 12.
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Saving gas

As seen in the table below, purchasing a 4cyl econobox or a 4cyl hybrid to replace your comfy 6cyl sedan would save a bunch of money every year. If you can afford it, it does make the best sense from an environmental point of view, but purchasing an expensive new car just to save $900 per year in gas is not an option many of us can afford. To most of us it makes m to keep driving our current gas guzzler. Modifying the sedan to get 25% better mileage, for under $1000 would start paying back after only two years. None of the modifications below in itself will provide a huge change in efficiency, but 3% here and 5% there all add up to big numbers eventually.
Vehicle
Configuration
MPG
Gas cost/year
Savings/Year
6cyl sedan
stock
26
$1615
$0
4cyl econobox
stock
40
$1050
$565
4Cyl hybrid
stock
50
$840
$755
6cyl sedan
aero mods
34.5
$1215
$400
using the 6cyl sedan as "baseline", and using gas costs of $2.80/gal and 15,000 miles/year

Installing a bed cover on your truck helps its aerodynamics. Air rushes over the cover and off the back of the truck, instead of into the bed, where it pushes on the tailgate and creates drag that can make you use more gas.
Roof racks are handy sometimes, but carrying things on the roof creates another source of drag that can increase your gas consumption. Keep your roof clear unless you actually need to haul something from one place to another.