www.fueleconomy.gov

realistically it depends on how you drive it.  When your boost gauge bars go up, get ready to literally watch your gas gauge go down.  I've gotten 23 to 25 mpg with mine already driving like my grandpa for a whole tank.  I also got 15 as well driving it another way!!  Who has 300 hp and don't use it?  

Ted nailed it on the head there, mines at 350hp, soo i rarely get over 20mpg, mostly my driving style. but most any car CAN get good mileage, just drive at lower RPM's

In my '04, I get just about 27 MPG day-to-day with mainly highway driving around Buffalo.  I got between 17 and  
20 MPG day-to-day in mixed highway/city driving when I lived near Baltimore.  On a nice road course, such as at
Summit Point Motorsports Park, I was seeing between 6 and 10 MPG.

16mpg depending how you drive it, extra orban 26.

18 or higher mpg

Like some have said...really depends on how you drive it.  I can get about 21-22 mpg if I drive it like a normal car (i.e., shift before 4.5 and keep the foot out of it).  I've seen as bad as 17 mpg while hot-rodding.  However, my commute is about 20 miles highway, and 10 mile in town.

yeah about 20mpg mixed is what I get, and my car weighs 2980. (with 1 gallon) and mods
I look at it this way, I think its impossible for any one to drive these things like grandmas all the time. its just to damn fun of a car to drive. when people get 20 mpg its from driving the car like a normal person maybe 85% to 90% of the time with on a few occasions hitting boost and driving a bit fast side. I mean its a freaking STI no one in there right mind would own one of these and never drive it fast once in a while.