By volume, alcohol contains 2/3 the energy that pure, 100% gasoline does. So let that be your guide as you crank out a comparison between E0 and E85.
The engine is designed for running regular gasoline. As such, it does not get as good of mileage from it. In order to do well with the E85, you would need higher compression than what gasoline allows. This dual fuel garbage doesn't work with propane, nor does it work with E85. The properties just don't trade off well, and mileage will suffer.
The trouble with that energy calculation Bill is that it is based on a BTU measurement ..... BTU stands for British Thermal Unit and is a measure of heat energy not Combustible energy - eg a stick of dynamite only has 2000 btu whereas a gal of gasoline has 125000 btu . Even though the dynamite has only 2000 btu it has an explosive energy many times greater than the gallon of gas..... It's all in the way you harness the available energy that counts towards the final result
There is a sensor in the fuel line that detects the level of alcohol in the fuel. I know there is always chatter about the corrosiveness of E85, but I have run E10 all of my driving life and have yet to see it. The tank on my 83 Trans Am is as clean on the inside as the day it was made, and I have run E85 through it as well. I also haven't seen any unusual deterioration of the fuel lines and seals, even on a car that had gone 5000 miles shy of 400,000 miles running exclusively E10 and occasionally some E85. I changed the injectors at 250,000 miles as they were simply worn out.