<span>When driving at speed of over 40 mph, one can increase the gas mileage by simply closing the car's windows. This is because at high speeds, open windows tend to reduce the car's aerodynamics which then leads to a significantly lower gas mileage. </span>
Since your car becomes much less aerodynamic if you have open windows when you're speeding over 40 miles per hour, you most likely should close your windows and, if you want, turn on your AC. If you're not going faster than 40 mph, though, open windows and no AC are the best way to increase your gas mileage.
