Unless the check engine light is on, there really is not much that can be done.
If the car were burning that much gas, the engine light should be on and trouble codes would be stored in the PCM computer.
Todays cars are very sensitive to changes in fuel economy.
Other that the normal maintenance
of checking tire pressure, accelerating smoothly, lessening the weight in the trunk, etc, the car should be running at the optimal fuel economy. If it were not, then the check engine light would be on.