I'd suggest displaying your Trip Computer on the Navi screen while you drive. As you do different things, keep an eye on how your MPG display responds. You could even try some simple hypermiling techniques to see how much of a difference they make. That's what I do, and it has helped me to recognize my own habits that either help or hurt fuel consumption.
Now, if your readout never goes into the 30 to 40 MPG range, then perhaps there is something that needs to be checked. On the freeway with the cruise control set to around 60 or 65 MPH, my display easily stays up in that range.
