i think you should be troubleshooting your car more, i.e.
EPA baseline MPG and the effect of E-10 - Tampa Gas Prices
"I had a car that ran good on regular gasoline but lost more than normal miles per gallon on E10. A small vacuum leak was the problem. Not enough of a leak to effect gasoline but just enough to keep the car from adjusting the fuel ratio to run good on E10. "