I travel from Oshawa to Airport Rd and Derry in Mississauga and back, every day (75km there, 75km back) in my ae92 GTS with 2 passangers in the car. If i gas up on sunday nite, I can probably go until Thursday Morning without filling up. Thats about 500-525km out of a 40L tank (75km X 7) Usually I gas up on wednesday nites with about a quarter left of gas. around $24 to $27 for a gas up if the tank is really low.
With my old 87 1.3L Civic HB I used to do about the same.. except to gas up I would pay about $3 less

. So for a heavier and faster car.. the GTS is pretty good on gas to me.

even if I drive it a bit faster than the Civic.