I can get 91 octane no ethanol gas here, or buy cheaper 10% ethanol 89 or 87 octane fuel. The 91 octane gives me 18 mpg in my 1-ton van, the 87 gives me 15 mpg. The 20% increase in mileage is more than the cost difference, so now the big van drinks the good stuff. (You used to be able to buy 87 octane no ethanol gas here, too.)
Got a new F150 4X4, 360HP, averaging 16.5 to 17 MPG on the midgrade.
Ain't tried the top grade in it yet.
Might take a shot next fill up.