Fuel economy (whether in an aircraft or a ground vehicle) is a function of negative forces being counteracted by positive ones.
In an aircraft, gravity must be countered by lift and forward drag countered by engine power (thrust). Thrust in excess of drag results in forward speed.
In a ground vehicle, gravity (weight) and its derivative (rolling friction - inherent drag) must be countered by a small amount of engine power and is required to keep the vehicle capable of moving forward.
The BIG demand on engine power is wind drag and desired forward speed.
Drag in the horizontal plane is a function of surface area being moved and the speed at which it is being moved. You experienced this as a child when you put your hand in the wind while your dad was driving. With your hand in a knife edge, there is little force on your hand and little effort is required to keep it straight out. Turning your hand towards the vertical (increasing the surface area exposed to the wind) caused your hand to fly back (and up - lift).
Wind in a moving vehicle is made up of two parts. The speed of the vehicle over the ground PLUS or MINUS the speed of the wind around it.
A vehicle with the SAME cross section in a no wind condition and a contant speed of (say) 60 MPH, has an inherent drag of a certain amount (we will call that drag "X" just to put a name to it).
That speed, still air drag, and surface area will require a fixed amount of engine power (and the fuel needed to keep that engine speed). That will be your best fuel economy for 60 MPH.
However is the wind is moving, that movement will effect the drag on the vehicle. If the wind is in your face (like your dad driving faster), the force on your vehicle will increase rapidly and require a significant increase in engine output to maintain that 60 MPH. Conversely, a tailwind (wind pushing on that surface area) will reduce drag (like your dad slowing down making it easier to hold your hand vertically) and significantly reduce the engine power required to hold 60 MPH.
For those who want the math, see the link here (http://en.wikipedia.org/wiki/Drag_(physics
)) however what you need to know is that drag is VELOCITY dependent as an exponent. Doubling your speed increases your drag by a factor of EIGHT. Tripling your speed increases drag by a factor of TWENTY SEVEN.
required to overcome the aerodynamic drag is given by:
Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower
(7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work
. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.
So, as in an aircraft, when traveling into a headwind, reduce your speed by the same amount as the headwind component to maintain your no wind fuel economy. Any speed above that "bug speed" will require an increase of engine power and a corresponding decrease in fuel economy.
Just remember that "CUBE" of the velocity increase from every MPH above "Bug speed."
With a tailwind, you will see a huge increase in fuel economy because the drag reduced will also be effected by the CUBE of the tailwind component.
Many of the seemingly unrealistic MPG claims can be laid at the feet of a particlarly good day with a particularly good tailwind.