This is a rather old theory and it is said that rolling the windows down at high speed will create an aerodynamic drag on our car. It’s supposed to reduce the fuel efficiency of our car. The predominant suggestion is that it is more efficient to turn on the air conditioning or use the internal venting, instead of rolling the door down.
It is clear that there’s some amount of truths in this assumption, but we should also consider latest scientific findings based on actual tests. There are studies performed to find out whether we could really reduce fuel efficiency by turning on the air conditioning. The tests were performed by Edmunds.com, an automotive website with orientation on researches and The Consumers Reports. These studies tried to compare whether opening windows or running the air conditioning has an effect in the fuel efficiency. The tests were performed on a variety of cars, such as SUV and sedan. Although the recommendations are nice in theory, they are not true in practice. The cold hard fact is that there is no measurable difference.
During the evaluation phase, researchers from Edmunds drove two cars in a 55-mile loop at an equal speed – 65mph. In the first loop, both cars were driven with the windows up and the A/C on. In the second loop, both were driven with the windows down and the A/C off. The tests were performed at long enough distances to ensure that the gas mileage level can be calculated properly. From the evaluation of these trusted independent sources in the automotive world, we could come with some hard facts. Conclusions can be reached independently and separately from each other.
It is found that using the air conditioning does draw some power from the car’s engine and thereby consuming some extra fuel. However, the effect can be negligible for short-range trips. The car’s fuel economy is reduced by approximately one mile per gallon when the air conditioning is turned on. The effect isn’t significant especially if the car is efficient enough and burns only one gallon for more than 20 miles of driving. The effect is noticeable only if we drive a gas-guzzling SUV that burns one gallon of gasoline for each 10 miles. Some A/C units are also more powerful than the other, so turning it off will further reduce the mileage.
Study also finds that opening the windows makes the aerodynamic drags higher on our car. However, it doesn’t seem to have measurable effects on the overall fuel economy, even at typical highway speeds. Also, the extra fuel we burn by turning on the A/C effectively cancels out the more streamlined shape of the car. However, it is likely to be true that we will save some fuel if we turn of the A/C during short trips in the city, especially if the overall speed is low. If the heat is not unbearable, it is advisable to roll down the windows, instead of turning on the air conditioner.