Saving fuel in the summer? Turning off AC might not be the answer

Humidity, Vehicle Speed, Recirculated Air All Come Into Play

With summer in full bloom and many Americans hitting the road, some drivers will try and lower their fuel consumption by opening their car windows instead of cranking up the car's air-conditioning unit. That may not be the answer to maximizing fuel economy, however, says SAE International. It all depends on speed and traffic conditions, as well as how long the car has been heating up.

Without a doubt, air-conditioning is "one of the highest energy consuming accessories" a car has, and its effect on fuel economy varies, depending on how fast the car is going, outside humidity, and traffic patterns. For instance, in some cases, the impact of open windows may bring down fuel economy faster than running the AC because they increase a vehicle's aerodynamic drag.

In general, using recirculated air for the AC system is more energy efficient than pulling in outside air but, again, it all depends on the conditions. For example, running AC at full blast can cut the fuel economy of a vehicle traveling at 30 miles per hour by as much as nine miles per gallon. The report suggests first opening the window to cool down a heat-soaked car, then moving to the AC blower. It can be complicated.

In the summer sun, a car heats up a lot faster than a house does, according to the SAE. During a 94-degree day, a single-story house that starts the day with an interior temperature of about 70 degrees can add as much as 10 degrees Fahrenheit of heat. In the car? That temperature can surge as much as 70 degrees. In fact, the surface of a car seat can reach 130 degrees Fahrenheit by late afternoon, while the top of a vehicle's instrument panel may reach more than 200 degrees. Whether you use AC or an open window, something needs to be done about that heat. Take a look at SAE International's press release below and you can find a link to the SAE report here.
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Think Turning Off A/C In Your Car And Opening Your Windows Saves Fuel - Think Again!

WARRENDALE, Pa., Aug. 5, 2015 -

Members of the SAE International Interior Climate Control Standards Committee authored an article discussing fuel consumption. The choice involves your and your passenger's comfort. There are many factors that impact how much fuel is used, including the specific vehicle fuel use (MPG rating) for the type of driving and the temperature and humidity conditions.

Consumers continue to be concerned about the fuel consumption of their vehicles. Many are not aware of the impact of accessories on this fuel consumption. Vehicle air conditioning (A/C) is one of the highest energy consuming accessories and has become standard equipment for vehicles.

The energy required to provide comfort in a vehicle depends on many factors such as the load on the A/C system is a function of the fan setting, the outside ambient and humidity, and the speed of the vehicle among other variables. The fuel consumption of the A/C will vary greatly depending on the weather conditions that prevail in a given area and also on the traffic patterns that are typical of a given city.

Comparing the use of a vehicle A/C system to a home air conditioning system may help consumers to understand why vehicle A/C can have such a large impact on fuel consumption. Consumers expect the vehicle A/C system to provide nearly instant relief after a hot soak in the summer sun. Home air conditioning usually runs continuously during the day, even if it may be set back at times to save energy. The demand and expectations of the consumer of the vehicle air conditioning system to provide comfort and maintain driver's alertness may require increasing the system cooling capacity resulting in larger energy demands.

Depending upon many factors, the energy requirements when operating or shutting off the A/C system and opening the vehicle's windows could be very similar.

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SAE International is a global association committed to being the ultimate knowledge source for the engineering profession. By uniting over 137,000 engineers and technical experts, we drive knowledge and expertise across a broad spectrum of industries. We act on two priorities: encouraging a lifetime of learning for mobility engineering professionals and setting the standards for industry engineering. We strive for a better world through the work of our philanthropic SAE Foundation, including programs like A World in Motion® and the Collegiate Design Series™.

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