Driving with windows down can increase aerodynamic drag, especially at higher speeds, which may reduce fuel efficiency.
At low speeds, the impact of open windows on mileage is minimal, but at higher speeds, it can significantly reduce mileage.
Turning off the AC can save fuel, but if you're driving fast, the drag caused by open windows might offset these savings.
In stop-and-go traffic, open windows with the AC off generally improve fuel efficiency since aerodynamic drag is less of a factor at lower speeds.
On highways, it’s often more fuel-efficient to keep the windows up and use the AC due to the reduced aerodynamic drag.
The impact on mileage can vary depending on the car's design. Larger vehicles with more surface area might experience more drag with open windows.
For optimal fuel efficiency, it’s generally recommended to drive with windows down and AC off in stop-and-go traffic and to use AC with windows up at higher speeds.