When we use to travel around Oregon and Nevada I would try to keep from filling up in California....  MTBE then Ethanol...  Good for 10% + reduction in mileage.  
Now Oregon and Nevada has been mandated to have E10...  Just sucks.
So our 94 f250 460..  with a 4200lb Bigfoot camper Gets 10 +-1 with E10....  Before MTBE and Ethanol 11-12..
Towing a 2Klb  toad or 2Klb trailer with Motorcycle n stuff it's 9... now with E-10, never under 10 with no MTBE or Ethanol.
There are some gas stations that state "UP TO 10% Ethanol"...  I can tell when the Ethanol is down or absent.... 
So in your case you might be visiting gas stations that might have the "UP TO 10%".
Such a lie, that our mileage would stay the same, we have had imposed on us.
One last word... We still have a 94 geo metro 3 cyl with AC, cruise at 70 all day and get 45mpg...  Ethanol...  40...  
Jim