My 2005 Focus ST 2.3L Duratec gets 25 to 27 mpg city and 31 mpg hwy (72mph) with the a/c on, and running 87 octane Shell.
I just installed the Steeda short ram intake and suspect it is improving the gas mileage measurably. Another long trip should bear this out.
There is a difference in the brand of gas you use. See: