My 2002 Gt gets around 20-21 mpg on regular 87 octane with mixed driving, and 22mpg on 93 octane, mixed driving also. Its also a 5 speed car.
Highway around 25-26 I would think, have not took a pure highway drive to burn a entire tank of gas in it yet.
Its actually great on gas I think, better then any other car I have had.