Colder weather will reduce mileage a little. If you let the car idle and warm up it will be even worse of course (0mpg when not moving)

First thing to check is the air filter as it seems to be the most commonly overlooked cause of decreased mpg(probably already have if did a tune up).

Next Id try some fuel injector cleaner myself, not only will it help clean up the injectors but you might have carbon build-up on the exhaust valves which reduces compression and allows the fuel & air mixture to vent past the valves before ignition even takes place(means more gas because you have to push the pedal farther just to get going or maintain speed).
Cars that burn oil, even if there's no smoke but a little oil use, will develop those blow-by symptoms much faster but any car can have some build up over time. Depends on the quality of gas you get as well, I stick to places like Shell or BP.

If a tank or two of that doesn't help noticeably I would check the ignition timing.