I don't own a 2008 Impreza so I can't tell you what MPG it gets. That being said I own a 2004 STI and I can tell you what MPG I get with my driving habits. I average around 18-19 MPG between my city/highway driving with this car. I admit I'm a bit hard on this car using the turbo boost all the time and shifting at between 4k-5k RPM. The way I look at it, if I wanted to get good gas mileage, I would have bought a Prius. That being said the STI is no longer my daily driver. My daily driver is an old Honda and it averages between 32-34 MPG city/highway with my driving habits. My STI sits still long enough nowadays, I have to add fuel stabilizer into the gas tank just to make sure the gasoline doesn't go bad.