I have been of the opinion that running air conditioner in the car can decrease its mileage but I realized that it was a myth when I read a book on how to Improve Car Aerodynamics. It is a really informative book and I came to know that aerodynamics of the car i.e. its capacity to steer through the air in speed also plays an important role in mileage of the car. It was suggested that when moving on high speed, one must roll up the windows and put on the air conditioner to improve the aerodynamics of the car. The open windows can act as a resistant in the drag of the car as the air which enters the car makes it heavy while speeding.