Hi
I have 2009 Honda Accord V6 (59k). For all 3 years I owed it the fuel consumption was about 24 mpg (city mostly).
Two weeks ago it dropped to 12.6 mpg. No check engine light is on. I use the same gas station. Air filter was changed in October. Tire pressure is normal. Car is running the same way. What can be a problem?

 

Check for brakes dragging? I don't know your mechanical abilities, whether you want to remove the wheels...? Or check another way.

Flat empty parking lot. Get the car moving at barely walking-speed. Put the transmission in neutral (or press down the clutch pedal if it's manual). It should coast smoothly.

You'll notice dragging brakes by the sudden grabbing at the very end of coasting to a stop. If you're not sure what it feels like, try again but rest your foot lightly on the brake pedal.

2009 is kinda new to be worried about a fuel leak from corrosion, but I guess it's always possible to damage a fuel line if something hits it under the car while you're driving. I don't know where you're located, but in wintertime it's not always easy to smell the leaking fuel.