Getting back on topic.... If your car sits for a long period of time before you use it and you dont run some kind of fuel treatment for the ethanol the gas breaks down and you get crap mileage as well.
Things to check when mpgs are low:
*Tire pressure
*Tire wear (looking for alignment problems)
*Brake calipers hanging up (touch the rotor if its blister your finger hot its not right, it should be warm/hot but not horrible - happens to me alot... mud lol)
*Spark Plugs - If the car sits they can get rusty/corroded from these crap fuels!
*Brand of gas - Sometimes smaller names like Marathon get better mileage than Shell
*Octane - Does your car require Premium and you put in regular 87? lol It happens Especially if the wife fills up the car lol ..... ughh
*Oil changes - when was your last oil change!!?? Running a full synthetic like Mobil 1 you can go 6000 - 8000 Miles before changing but running cheaper oils you will need to change every 3000 - 5000 miles to keep your motor in top shape. Lots of people get dollar store oil and wait 8000 miles before changing it and wonder why its like sludge in the oil pan....
This is a good start... There are other factors as well but thats a pretty good check list... a sudden huge drop I normally find I had mud in my brake caliper pad slides and it caused the pad to hang against the rotor ... take apart sand, clean, re lube and re assemble..... went from 9.5 mpg to 14 mpg one road trip!!!!!