An air tank you say...
Lol, I have no doubt in my mind that you are not going to be doing this. But just think of the aero improvement!
But in all seriousness you are doing a fantastic job with this Suburban. It boggles my mind that you are getting such awesome mileage with it. I mean, it absolutely smokes the mileage that I am getting in my Bravada (which is just a glorified S10 Blazer). If I could regularly pull down tanks over 20mpg in it I would probably keep it but with it's average being closer to 18 even when hypermiling I'm just over it.
Oh and what is the air tank actually going to be for?