STANFORD UNIVERSITY SCHOOL OF EARTH SCIENCES - EARTH SYSTEMS PROGRAM

Sustainable Choices

At Low Speeds, Open Windows & At High Speeds, Use AC

Simplicity:
pip pip pip
Carbon Impact:
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Money Savings:
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Health Helper:
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Overview
The choice of car cooling technique is a tricky one. While the question of air conditioning (AC) versus rolled-down-windows has not yet been conclusively answered, we can be confident in a few things. First, air conditioning does dramatically reduce the fuel economy of your vehicle at any speed. Second, lowering your car windows also reduces fuel economy by increasing drag. This drag becomes worse as your speed increases. The question remains: at what speed does the drag overcome the gain from not using your air conditioning? A study done by the Society of Automotive Engineers found that cars were more efficient with the AC off and windows down for all speeds up to about 65 mph, but that the margin of difference narrowed at higher speeds. So, for now we recommend rolling the windows down below highway speed and using the air conditioning while on the highway.

Tips & Tricks
Go without AC when possible. Air conditioning hurts your fuel economy, so if you’re not too uncomfortable without it, keep it off.

Roll down the windows while driving around town. If you’re traveling below highway speeds (for instance, stop-and-go traffic at a calm pace) rolling the windows down will save gas.

Use air conditioning on the highway. As you get up towards highway speeds, the drag caused by open windows begins to counteract the savings made by keeping the AC off.

Use AC in short bursts. If you’re boiling up and need some cool air, try doing bursts of air conditioning, maybe one minute out of every ten. Then re-circulate this cool air through the vents.

Web & Print Resources
About car air conditioning:
www.epa.gov

Articles:
www.bankrate.com
www.treehugger.com
www.grist.org

Study:
www.sae.org

Fuel-efficient driving tips:
www.fueleconomy.gov
www.edmunds.com
www.shell.com
www.consumerenergycenter.org

Fun Facts

Car air conditioners in the U.S. commonly used to use CFC-12 as a refrigerant, until 1995 when CFCs were outlawed because of their damaging effects on the ozone layer.
www.epa.gov