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was told only 8%, but since I do more highway driving, wouldnt that kinda be better due to the higher speeds? I could see city driving go way down...
That is a rough number based on ethanol having 20% less energy than gas and assuming that you would endeavor to run full E85. E50 would be closer to 8%. However, you may find yourself in boost more often with the e than you used to. If you are good at tuning, running some good timing advance can offset some of the mileage drop potential of ethanol.
Speaking for myself, I've had to raise the injection pulse by a little more than 10%. When mixing gas/E85, the ethanol content can vary (by station) depending on how much is added to the gas. So you need to pick a station and use it exclusively, if possible, for best results.