Most aren't aware, but the winter fuel formulation has a bit less energy per unit of volume than the summer formulation.
Now, back to the V8 discussion.
@Marcus85 makes a very interesting point - that there may be more behind corporate engine decisions than the fleet mpg pressures imposed by big government. (See what I did there?) The design challenge - and it's a big one - would be to get V6 mileage out of V8 powerplants. That's a challenge that might drive some very interesting R&D, but there's a simple way and cheap way to do it: small displacement V8s. About 10 years ago you could get a Dodge Durango with a 4.7L V8 that had (oddly, I think) 2 spark plugs per cylinder. It was reliable, but it got terrible fuel economy - worse than my 6.2L, 6,000 pound Raptor by far.
Now suppose, and this is just supposition, Ford took the venerable 5.0L V8 and put dual (separate exhaust and intake) valve timing and direct injection on it? Done right, with a good transmission, good differential ratio, and decent intake and exhaust components, I bet a truck so equipped could approach 20 mpg average.