Well, the air dam doesn't appear to be hurting anything. I had to go to Springfield today, ~80 miles on I-72, cruise at 55 both directions, set-it-and-forget-it, temperatures in the 50s. The way over:
And back:
Interestingly enough, comparing with this chart from Wayne Gerdes:
...my average today, 66 mpg, was 6.28% better. Figuring 2% drag reduction:1% fuel economy improvement, that suggests a drag coefficient 12.56% lower than stock, or .2186--not including any temperature compensation.