I got a CEL a little before halfway through the last tank, and it cleared in about 50 miles or so. Stayed off for the rest of the tank.
I decreased the amount of ethanol in this tank as a result. I think 4 gallons of E85 per tank is the sweet spot, the most that can be used without risking a check engine light. I put a little more than that in this tank.
I got about 269 miles before the low gas light came on, about 11 miles less than the all gasoline tank. That's a trivial loss in mileage.
gallons e85: 4.182 @ $1.559/ gallon
gallons 87 octane gasoline: 8.469 @ 1.859/ gallon
ethanol content of tank: 36.27%
octane of tank: 92.988
savings per tank compared to premium @ $2.559/ gallon: $7.58