Ok, now from what I've been told with talking to both FlexTek and Change2E85 I'll be losing about 12% of my fuel economy.
Hopefully, it will only be about 12% Granted, a lot will depend on your driving habits as even regular gas MPG's can vary depending on how heavy your foot is. But, everything I've read(and I admit I haven't done as much research as you) states that E85 has about 75% of the energy capacity of gasoline. That should equate to a 25% increase in fuel and also explains the need for higher flow fuel pumps.
On the other end of the spectrum is that fact that E85 is also a higher octane, so perhaps E85 is able to make the same power without having to use 25% more fuel and that's where the 12% comes from.
I still think though that 12% is a bit of a stretch. If it were me, I'd shoot for 20% and run the math that way and see if it still makes sense and if it ends up being 12%, that's just gravy.
There was an article in one of the major car magazines in the last year or so where they did a comparison running a flex-fuel vehicle on 100% gasoline and also 100% E85 and compared the fuel economy. I'll see if I can dig it up. Might give you some additional info.
All that said, I still think it's a great idea and the initial cost seems well worth it for anyone who owns a gas guzzler.
Will you still be able to run gasoline as well, or will it be converted to purely E85 use? Sounds kinda dumb, but maybe the computer override won't allow the ECU to compensate when regular gasoline is used? Just curious in case I look into this in the future. It would be nice to have a flex-fuel option in case you travel and E85 isn't available.
Edit: Here's a story that may be helpful. It's not the exact one I was looking for, but it's something. It compares all sorts of gas alternatives not just E85.
http://www.motortrend.com/features/consumer/112_0705_alternative_fuel_technology/index.html