By Hoffman Auto Group | Last updated Sep 9, 2011, 8:02 am
Written By Ron Winter
For decades automobile owners have debated whether air conditioning impedes gas mileage to a degree that it shouldn't be used, or whether the open-window "flow through" method of cooling is just as bad.
"It's a question we get a lot from people, especially during times of extreme heat," said Shawn Pinto, service manager for Hoffman Nissan. Pinto has been with Hoffman for over 10 years, and has experience with American, Japanese and German vehicles. "Drivers are always looking for ways to boost gas mileage and save money. Air conditioning use versus windows down is one of the longest-running debates for automotive experts."
Shawn Pinto
The argument goes that the air conditioner puts more strain on the engine because of parasitic draw when the air conditioning compressor is engaged, thus requiring extra gas to keep the vehicle at a constant speed.
Auto air conditioning proponents have long countered, however, that when a vehicle's windows are open the reverse pressure encountered when the air stream runs inside the car and up against the rear window also cuts down on the miles traveled per gallon to a degree that is comparable to using the air conditioner.
Recently a limited test on certain automobiles concluded that due to advance aerodynamic designs, running the vehicle with the windows open results in a negative impact that far exceeds the drain on the engine when the air conditioner is used.
The Society of Automotive Engineers tested both a full-sized sedan and a sport utility vehicle on an open track and in a wind tunnel, and as reported by HowStuffWorks.com, concluded "driving with the windows up and the air conditioning on is typically a more fuel-efficient way to drive."
SAE reported that "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. 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. The study concluded that the more aerodynamic the vehicle, the more drag open windows will create.
Since only two cars were studied under fairly limited driving conditions the results can hardly be considered absolutely conclusive. Nonetheless, the study does show that there is a significant disparity in fuel usage between a sedan and a SUV when comparing air conditioning and open windows - but in favor of using the air.
"Air conditioning versus windows down is a hard question to answer because there are so many variables that need to be accounted for," said Pinto. He explained that speed, weather conditions and the type of vehicle are all factors that contribute to the fuel efficiency of either AC or natural air. "The short answer to the debate is that maximum fuel efficiency, while staying cool, depends on the situation."
There are myriad arguments against using air conditioning, with some drivers preferring to purchase much smaller cars and hybrids - for their improved aerodynamics over SUVs and trucks - but often the issue goes much further.
According to the SAE study, driving at speeds in excess of 55 mph with the windows down result in a dramatic decrease in fuel efficiency - as much as
20 percent, versus around 10 percent with air conditioning. For this reason, many people refuse to drive over the old 55 mile per hour speed limit, even if they are in areas with 65, 70 or 75 mph speed limits. There are also some who drive only a night, or in the early morning and early evening hours to take advantage of cooler temperatures.But aside from the speed issue, it is likely to be difficult for most people to alter their driving habits to such an extreme, in the interest of a potential savings of a few gallons of gasoline. And while driving a smaller vehicle or a hybrid certainly is a commendable effort, there is no way that approach will work for families with children.
Driving a long distance with an infant or toddler along for the ride can be difficult enough without also applying the extra burden of extreme discomfort. And there are reasons why some drivers prefer trucks, SUVs and full-sized sedans, including all the gear that growing families need when they take part in school sporting activities, summer vacations including camping, or carrying tools to the job site.
But if achieving maximum fuel efficiency - while staying comfortable - is situational, in what a setting do you choose to blast the AC?
"Highway driving is the most fuel-efficient use of air conditioning," said Pinto. "When you're driving around town, and having the windows down keeps you sufficiently cool, then don't use the AC."