Avoid using AC at low speeds.

While it is much better for the fuel efficiency to turn on the air conditioning instead of opening a window while driving very fast on the motorway, it is usually not the case when travelling at a slower pace. The reason for this is that at high speed the air turbulence caused by opened window when driving fast impairs the fuel mileage more than the air conditioning. But when you have a slow ride around town, you should consider turning the AC down and opening a window if it is not too hot outside to do so.