Based on a study conducted by the Society of Automotive Engineers (SAE), driving with the windows up and the air conditioning on is typically a more fuel-efficient way to drive. We'll get around to discussing when air conditioning isn't a more efficient option, but let's first take a look at what the SAE found.
The SAE study was conducted at a General Motors wind tunnel and on a desert track. In the wind tunnel, air was forced over the front of the car and also from an angle on the front of the car to simulate a cross wind. In the desert, temperatures and vehicle speed were factored into the study. Two vehicles were used in the test, one was a full size SUV with an 8.1-liter V-8 engine and the other was a full-size sedan equipped with a 4.6-liter V-8 engine. Overall, both studies showed that driving with the windows down has a significant negative effect on the fuel efficiency -- more than using the vehicle's air conditioner.
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. 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 90 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.
So, when traveling at speeds around 80 kilometers per hour or faster, air conditioning is usually a better bet.