Now, the rest of the story--
Last summer, I measured my fuel economy very carefully. My "test track" was I-80 from the Toole truck stop near Salt Lake City to the Wendover, Nevada, exit and return. Distance, very close to 200 miles, almost perfectly level. Mileage was very close to 65.5 mpg at 60 mph. Speed and distance were checked with a GPS.
I just finished calculating the mpgs, assuming no wind, 4500 ft elevation and 70 degrees. Here are the input numbers--
Frontal area...............23 (Uhh, I guessed that number)
Drag coefficient.........0.31 (my measured value)
Air density................1.01 kg/cu.m.
Weight....................3050 lb (measured, with me + full tank)
Rolling resistance......0.0065 (my measured value)
Fuel density.............3200 gm /gallon
Engine efficiency.......Empirical fit to published data for my engine
Transmission eff........0.85
The program gets 65.56, which is too close to be believed.
That all looks very nice. But, if I plug in the factory Cd = 0.38, I get about a 6 mpg drop, which doesn't match my measurement.
/Ernie
Quote:
Originally Posted by Ernie Rogers
This is a hard question for me. The reason is I'm not 100% sure. [When asked about the benefit of the wing.]
Shortly after building the wing, I tested it very carefully and got a 3 mpg improvement no matter what my speed was. (At 60 mph, I got 58 mpg without it and 61 with it, as I recall.) This actually makes some sense because at higher speeds, the 3 mpg represents a bigger gain.
Later, I measured my Cd and Crr (rolling resistance coefficient). Armed with this and some other car parameters, I can calculated miles per gallon a-priori.
I will check my figures on this and write a little later, I just got a call for dinner. 
Ernie Rogers |