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How many miles are on the vehicle?
the most common problems to affect mileage would be a dirty air filter and low tire pressure.
more common is just the type of driving you do. My wife had a jeep liberty once we were only getting 11mpg average. these are not good mileage vehicles anyway but it should of at least gotten 15-17mpg!!! i replaced plugs filters injection cleaning every trick I could think of to help. Finally one day i drove it on her route taking kids to school and all. The problem was as soon as it could shift thru the gears and get cruising it was time to stop again for a light or bus. So the vehicle was never able to get to its sweet spot to not gulp fuel. I know this is unrelated but my point is it was not the vehicle but just the style driving that didn't match up.
what kind of driving do you do mostly?
The routes taken are mostly stop and go like you describe. However even driving on highways it seems like you can even watch the gauge go down in real time. We have had many different kinds of cars with similar driving routes but this car is noticably different. It is just about as bad as my previous vehicle which was a Silverado 2500HD.
I would check the spark plugs if it had the wrong style installed at some point this could affect it. bigger tires also. it is not due for plugs till 120k miles but somebody may have installed a different type. Also be sure no one has messed with the intake air temp sensor. places sell a "chip" that is just a resistor to trick the computer into thinking it is 0 degrees outside and richening fuel mixture to give more power. the iat sensor is built into the mass airflow sensor this is mounted in the air intake look to see if wiring has been tampered with.