From the rear valance, my design extends back 26" to the rear subframe and is nearly 4ft wide. From the subframe, the underpanel begins a gentle rise at the same angle another 12" back. So we are talking about nearly 12 square ft of area acting on the air.
A and B of diagram above, the best angle for drag reduction (CD) is from 2 and 4 degrees.
A (like mine), the best angle for reducing lift (CLR) is 6 degrees.
B, the best angle for reducing lift (CLR) is 11 degrees (at the expense of increased drag)
Made out of black coroplast, total weight with strakes, just over 4lbs.
Visual indication of how far back the 6deg rake goes back to combine with the flat underpanel:
Now for the shocking results!
I drive the same commute to work everyday and usually average about 26-27mpg. After logging 106 miles with the diffuser, I filled it back up with 3.538 gallons....that's now an avg of 29.9mpg!!!
Joe