Aerodynamic Drag
Aerodynamic drag is the force air pushes against a moving car, making it burn more fuel to keep going.
Think of it this way
A flat tray dragged through water feels the water push back hard against its wide face. A small round ball moves through the same water with much less effort. The tray has more surface facing the flow, so it slows down more. A car with a smooth, round shape cuts through air just like that ball cuts through water.
Explaining aerodynamic drag by grade level
When a car drives fast, air pushes back on it. The car has to work harder to move. That uses more gas. Driving slower means less air pushing back, so the car uses less fuel.