[MetroMPG]

All else being equal, cold temps will definitely drag down your MPG.

You could address that somewhat with a block heater and synthetic engine oil (more stable viscosity than dino, so less of a warm-up hit at start-up).

Do you have instrumentation?

Also: look up user Thymeclock. He's got an Aveo automatic, and knows quite a bit about the operation of the transmission. I believe he'll tell you getting good MPG in the city below a certain speed (before the torque converter locks) is particularly challenging in this car.