The short trips and city driving are the real killer
my last fillup was strictly 5 mile trips or lower... until the last 40 miles which was a trip 20 miles each way. Now before I left for that trip looked at what the gauge said and it said it was 15 MPG. After the 40 mile drive it went all the way to 20 MPG. Granted the gauge is 1 or 2 MPG little off but still those 40 miles at the end really brought it up. I was going 80 cruise control on with a few WOT runs. I wouldn't say that's bad at all.
for those interested heres my fuelly
http://www.fuelly.com/driver/mcwargod/impreza