While I haven't actually done an actual mileage tracking calculation, I pretty much have figured out why Auto Start Stop exists and why the V8's mileage is reported as so low. I don't understand why they do it this way, but it appears that when you are idling, it counts as a constant value of "0 mpg" which gets added to your mpg average totals. I can only assume that the EPA requires auto makers to do this, which is why the auto stop really would make a significant difference in reporting, but not really in actual mileage.
I figured this out because I was curious as to why I would be driving down the road in 4 cylinder mode and I would consistently see mpg's around 24mpg on a level road, but my average mpg was so low, so I decided to reset the mpg and test it out. I was driving down the road with an average of 24 mpg, and then I stopped a light and my average mpg was falling down very quickly right in front of my eyes. So, basically every time you remote start your car, or sit at a light, or are stuck in traffic, your average mpg is going down dramatically. For somebody like me who doesn't do a lot of long distance driving, this makes a big difference in what the average mpg gets calculated to. So many experts I have seen have said that the auto stop really doesn't save much, but when you stop all of the idling counting as 0's, you can see where it would make a big difference, at least in what the car is reporting to you.
I don't agree with this methodology of using the 0 figures in the average calculation. While it's true that yes your engine is running and you're getting 0 miles for it, it is not a fair rating of what kind of mileage the engine is capable of delivering. Every gas engine regardless of efficiency is going to get 0 mpg when it's not moving, so I feel that it would be more useful for the car to simply delete 0 entries from its average ratings so that you'd get an average of mpg based on the engine actually doing what it's supposed to be doing. I now no longer give a crap about what the car is reporting as average mpg because I know it's not right.
Eventually I'll take a road trip and calculate my mileage and see how it goes, but until I do I'm not going to regret my hemi, especially when I'm merging into traffic...