I filled up at Meijer yesterday afternoon (the low tank light went off right in front of Meijer! Sweet!). Here are the stats:
Price E85: $3.62 a gallon
Price 87 Octane gas: $4.12 a gallon
Gallons E85: 6.007
Gallons 87 Octane: 8.009
Gallons left from tank #10 (@~35.152% ethanol): 2.184
I'm assuming that the E85 is 79% ethanol, 100 octane.
Ethanol content of tank #11: 38.501%
Octane of tank #10: 92.488
Tank 10 finished with a mileage of 26.1. Average speed was 35 mph.
At 38.5% ethanol, this tank is well over the ethanol content that caused the CEL to light on tank #3. At 35% ethanol, tank #10 had almost as much ethanol as tank #3, but no CEL lit.
And as commente Alcohol likes to mention, my estimates of E85 ethanol content is most likely conservative. So I probably have more ethanol in the tank than I have calculated above.
It seems like you need to SLOOOOOWLY build up to higher and higher levels of ethanol. I probably got the CEL on those early tanks because I went from 30% to 40% too quickly.
My strategy now is going to be to increase each tank by half a gallon of E85 each time I fill up. That's why I went from 5.5 gallons of E85 last tank to 6 gallons on this tank. Next tank will be 6.5 gallons.