The MPG site isn't "my" mpg site, and it pretty much pegged the mileage you achieved at the speed you were going, within 1mpg.
Funny how that happened.
I'll deduct from that you don't like the site because it confirms your higher speed driving is killing your MPG's. Don't kill the messenger.
I can only see that this thread would be valid if the Impreza was not meeting the Fuel Economy numbers printed on the Window Sticker, numbers derived from certified EPA testing.
If you look at your Window Sticker that came with your new car, you would realize that your car is capable of achieving the numbers on the label.
You can further confirm this by going to Fuelly and looking at the accumulated data from many drivers -- The average driver is hitting the estimated Combined Fuel Economy.
How many people are complaining without knowing what the Window Sticker actually shows, and what those numbers mean (including the Expected Range For Most Drivers numbers under those big bold numbers).
Why are you comparing it to other manufactures cars? That isn't how the EPA numbers are derived, and the numbers on your sticker are accurate for your car!
So why is this discussion in the Warranty Complaint thread?