Not trying to be an a**; I just know from trying to do experiments in the field that there are many, many variables to contend with that can make the results unreliable or even useless, depending on how great the environmental variations vs how minute the test subject values are. .
How much effect does wind direction have on fuel economy using a 2006 Honda Civic Hybrid (HCH II)?
Outcome: There was an increase in Fuel Efficiency of 18.7% with tailwind
Very good! Aside from all the "criticisms" you're gonna get, you proved several things: 1) Wind helps / hurts MPG!
2) Your observation of wind speed (5-7 MPH) gives an average value of 6 MPH ; The total delta is ~12 MPH.
3) Don't forget these values are for your Civic only! Other shapes will act differently (ignoring such parameters as differences in weight, frontal area, accessories, wax jobs (LOL), etc.).
4) The choice of a writing utinsel beats a stick in the dirt every time! LOL
In my xB on my trip to Maine I was cruzing at 70-75mph behind a few big vehicles and was getting 40-42mpg then lost them and it dropped to about 32mpg. This box is definately not like my Geo.
You found out a truth! Drafting can be fun! Also dangerous! A rock kicked up by the rear tires on these big rigs (most mudflaps are too short!) will find your windshield / paint job / headlight! Bang! Then, think of all the gas you saved!
-Been there, done that, no more! I stay away from trucks! Ted Hart