Tonneau covers improve your gas mileage by sealing your pickup truck bed from this airflow, which improves the aerodynamics. A tonneau cover will help reduce aerodynamic drag and can improve gas mileage. It will cover your truck into vault and your bed will generate less air drag as compared to open bed as showing in the pic below. This will increase your mileage by around 5%, some vehicles area also able to save even 10%.