For the sedan, when the windows were down, the efficiency was reduced by 20 percent, while the SUV fuel efficiency was reduced just 8 percent [source:
Hill]. These differences are an important factor in determining just how much the windows down option will affect the fuel efficiency of your vehicle. The study concluded that the more aerodynamic the vehicle, the more drag open windows will create.
When driving at speeds of more than 55 miles per hour (88.5 kilometers per hour) with the windows down, there's a decrease in fuel efficiency of 20 percent or more. Although using the air conditioner decreases fuel efficiency as well, cooling the air through the compressor only decreases the fuel efficiency by about 10 percent [source:
Arthur].
So, when traveling at speeds around 50 miles per hour (80.5 kilometers per hour) or faster, air conditioning is usually a better bet, but what about when you're simply cruising around town? Keep reading to find out how you can save a little bit of fuel on those slow-speed short trips.