I have a 4.7L flex fuel. I conducted an extremely unscientific couple of tank average using E85 (85 octane ethanol). I know it is not a direct comparison to fossil fuels, but here's what my averaged numbers at the time told me -
A tank of E85 did not give me as many miles per tank, but was about 30 cents a gallon cheaper.
A tank of 87 octane gas got me more miles per tank, but cost about 30 cents a gallon more.
I don't have my numbers anymore, but after the math, my price per mile for fuel turned out to be statistically the same, within the margin of error for my methods. Since I don't like to stop at filling stations, and the E85 station is not easy for me to get, I almost exclusiely burn 87.
While towing, I have noticed that I don't seem to have to bury my foot in the accelerator as much climbing hills with 89, so occasionaly I splurge there. I have never run real numbers on MPG doing that, it is just an opinion based on the subjective feel of the gas pedal.
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