"If your guage doesn't show "full" after a fill try leaving the ignition switch "ON" (not running) during the fill...works for me every time and I've made it a habit of doing this because of the problem you stated.
We average 38-44 MPG. Best was 48.4, worst was 32. Ours is about 50/50 hwy/city. Usually use auto mode and shift to neutral coasting to traffic lights and when waiting for the green light. Currently have just over 16,000 miles in one year. more miles seem to equal better MPG.
Finally got my car in January. Other than wearing a thick coat of Minnesota winter grime all over it, seriously learning to dodge potholes, and remembering how to drive a rear wheel car in the snow, my Smart and I have been having a ball.