2014 Ford Focus SE (flex fuel) mpg is horrible?

So we got a flex fuel rental car and the first week we had the rental was great. We didnt have to fill up at all. After the first week i accidentally put regular gas (about half a tank) and the mpg was terrible, it barely lasted a day. The second time i filled up (to full) the tank was almost empty and i used E-85. It lasted about two days and i only got 23.3 miles. Im still using the E-85 and getting about 23 mpg.

I dont know what the rental place put in the car, but how can i get my gas mileage back up to what is was during that first week. Should i try regular 87 gas again but use a full tank? Any suggestions?