On the diesel at least I'd say the air dam has very little impact on mileage. Given that the second trip had just a little more highway and a little less city it's possible the mileage was hurt a little but I doubt it was even 1 mpg.
I wondered if the diesel, being heavier and 'torquier', would see less of an impact with the removal of this air dam, which I intend to do when my truck is finally built/delivered.
Heavier: Less 'nose rise' at highway speed compared to gas, so less wind-scooping.
Torquier: Small resistance variances from wind buffeting would be absorbed better, meaning less rev to compensate = less fuel needed to maintain speed compared to the gas versions.
This is totally a theory, and I could be completely wrong, but...
I am also toying with the idea of simply trimming a couple inches as opposed to removing the whole thing. The way I see it, letting it hang a bit near wheels, as an above poster mentioned, may provide a bit of a wind break, now without the clearance issues. The only thing is you also then get rid of that lip at the bottom, which I'm sure acts as a mini spoiler to the spoiler. If cut, it would be flush at the edge. Of course, this could also now make it act as a wind FUNNEL, thus amplifying wind resistance...
I really could care less how it looks, it's the fact that it takes an off-road vehicle and makes it no more capable than a minivan with it on.