I noticed the same issue on a 2011 Tahoe, I owned. It had an air dam and a built-in factory "rake/stance" (factory tow package) where it sat with the rear end higher than the front. I put a leveling kit on the front to even out the rake/stance, and removed the front air dam. It got about 2-3 mpg less at cruise on the highway. The aero pkg on these trucks is really important if you do a lot of highway driving....

It's spelled dam.