Don't want to throw a wrench into this - but isn't this going to increase frontal area quite a bit? The big question might be this - is the underbody drag greater than the increased drag from increased frontal area? Why don't you try the coast down test with and without that air dam on there and let us know the results.
Here's the Thread for the Coast Down Test Process
That 2.3 mpg increase you had... while it sounds great - it is sooo hard to absolutely attribute it to your mod. It could have been anything else in the last tank - from routes, driving conditions, accel rates, etc. To be scientific about it - you have to isolate it. This test could do that...
I've often thought of putting on an air dam, but every time I think how much frontal area affects Cd, I freak out and decide against it. Would love to see a before-after test of this concept. It would help me, and I'm assuming many others, to take the leap.
