Unless you're calculating by hand you're doing it wrong. Also I can only assume you didn't buy the car for it's stellar gas mileage, so not sure what you're expecting. By hand I get about 19mpg city driving, so it sounds like you're doing fine. One other thing, you're probably not shifting at the right time if you shift at 2750 all the time. Your car is most efficient when in vac and becomes less efficient as boost rises. For best mpg stay in vac and shift when you go into boost. You're throttle position can also effect when you go into boost.