I have a 12V Viair 2 gallon tank system on my Silverado. I mounted it with a bracket I welded up on the passenger side between the frame and the side of the box. 150psi, and I believe about 4cfm.
Runs my train horns, and with the T and connector installed will actually run my 1/2" SnapOn impact well enough to break the lugs loose on the truck/trailer, run a quality air ratchet, etc. Its not intended to run air hog items by any means, but works great for my needs. (you can actually run a highspeed cutoff wheel for short bursts!)
Comes in real handy for adding air to tires, AirBorne hitch, etc too.