My experiences are similar to yours. My commute is short (15 minutes), but our weatehr has been relatively mild (only a few days near zero degrees F, most of the daytime highs have been 30 F or above). My first tank was 39.6 MPG but seems to be improving with each tank. I am on my third tank and getting around 44 MPG.

The rest of my commute is urban with hills and poorly timed lights. I don't use the heater too much but do so when I get cold or the windows begin to fog.