generally, with straight E85 my mileage went down from 17 to 13.
but i had one magical tank of E85 that got 24mpg with 2 other people in the car for 150 miles. i know that one tank was an anomaly- mostly 80mph on the interstate with the AC cranked. it makes no sense, but that's what it computed out to.
i have discovered that a 50/50 mix with 87 octane gives me the same mileage as straight gas, but a whole bunch more power.