Friday, July 31, 2009

Air Conditioning or Windows Down?

A debate rages about (who would guess?) which uses less gas: rolling down your car windows or turning on your air conditioning. However, all 3 tests I could track to their sources showed that running the air conditioner almost always uses more gas than rolling the windows down. For example, Car and Driver did a test on a sedan (they don't say what model, but the photos are of a small sedan) in which running the a/c reduced fuel efficiency by 15% at 35 mph, whereas opening the windows decreased it by only 1.5%. At 75 mph, running the a/c decreased miles per gallon by 6.8% vs. 2% for windows, and results at 55 mph were in between (losses for a/c decrease at higher speeds because the engine is producing more power.)

Consumer Reports findings for a Camry were less dramatic: using a/c at 65 mph decreased the car's fuel efficiency by 1%, whereas rolling windows down at that speed had no measurable effect on fuel economy. A third study by the Society of Automotive Engineers shows that while the results are closer for a sedan, both the sedan and the SUV used more gas with the a/c on than with windows down at speeds of 30, 50, and 68 mph when the temperature was 86F. Bottom line: minimize your use of air conditioning, especially when you're tooling around town. Keep in mind, too, when you ride the bus or train, you can enjoy A/C guilt-free, and bicycling creates its own cooling!

Comment with your thoughts, and stay cool!


  1. Huh, I always thought it was inefficient to open windows, especially for highway driving.

    I was riding around in a cart at the airport and surprised at how much cooler that felt, compared to walking. The movement generates a breeze, I guess.

  2. Opening car windows does usually reduce efficiency, but not as much as running the a/c, according to these studies, which were the only ones I could find. Moving air evaporates moisture from your skin, which cools it due to the energy needed to convert water from a liquid to a gas (same principle as the swamp cooler in last week's post).
    Thanks for your comments!