I filled up at Meijer yesterday. Here are the stats:
Price E85: $2.94 a gallon
Price 87 Octane gas: $3.44 a gallon
Gallons E85: 4.002
Gallons 87 Octane: 9.830
Gallons left from tank 4 (@~30% ethanol): 2.568
The difficulty is that we are in the transition from winter blend E85 to summer blend. The ethanol content of E85 at this time is unknown. It can be anywhere from 70% to 76%.
Worst case, this tank has a little less than 28% ethanol. Best case, it's 29%.
Octane would be similarly difficult to figure out. I think that, worst case, it is a little over 91 octane.
Tank 4 finished with a mileage of 24.5. So far, I've done about 15 to 20 miles of local driving, and the mileage is right at 20.