Using windows or AC to cool your vehicle in the summer has been an ongoing argument. But the answer has finally come out: there is a time and place for each. Running your car's AC is the main contributor to reduced fuel economy in hot weather. Its effect depends on several factors, such as the outside temperature, humidity and intensity of the sun. Under very hot conditions, AC use can reduce a vehicle's fuel economy by more than 25%, particularly on short trips. So, limit its use to highway speeds. However, driving with your windows down can also reduce fuel economy. Open windows increase aerodynamic drag (wind resistance), making your vehicle use more energy to push through the air. So, only roll the windows down at lower speeds.