Thursday, June 10, 2010

Apple #462: Windows Up and A/C On or Windows Down?

I'm about to leave on a road trip for the weekend. So I want to know, does it use less gas to drive with the windows open, which probably creates more drag on the car and thus requires more power from the engine, or is it more fuel-efficient to run the air conditioning, which also requires more fuel?

WIndows down or A/C on?
(Photo from Climate Clean)

Answer: It depends how fast you're driving.

  • Air conditioners in cars, in general, reduce your miles per gallon by about 10% no matter how fast you're driving.
  • Driving with the windows down does increase drag on the car, but that doesn't become significant until you reach speeds of 45 mph or so. In fact, windows down at speeds greater than 55 mph will reduce fuel efficiency by 20%.
  • So this means that when you're driving around town, under 45 mph, it is more fuel-efficient to keep the windows open, A/C off.
  • When you're on the highway, roll up the windows and turn on the A/C.
  • Either way, you stand to lose a minimum of 10% in fuel efficiency.

Hanging your arm out the car window also increases drag. Older cars were actually designed to allow you to do this classic lean. Newer cars that are required to have firmer side impact ratings and all sorts of safety features have higher side panels and they're often slanted, which makes doing the arm-lean much less comfortable than it used to be.
(Photo of Matthew McConaughey in Dazed & Confused from extrawack)

    • That mph cut-off might vary depending on what type of vehicle you have. An SUV which is already really chunky and large and not aerodynamic is going to see an even greater drop in fuel efficiency, and at lower speeds, when driving with the windows open.
    • You can help your A/C work better if, when you first start up a car that's gotten hot in the sun, you roll down the windows, open the vents and run the fan to blow as much of the hot air out of the driving compartment. The A/C will thereby have a lower temperature to try to overcome.
    • Once you're driving and the cabin has cooled off, if your car has a recycle-inside-air button, turn this on. This recirculates air that's in the cabin and already cooled off, rather than pulling in hot air from the outside.

    The recirculate air button in Volvos looks like this.
    (Image from the 2002 Volvo S60 owner's manual)

    • The most efficient option of all is windows up, A/C off. But on an 85 degree summer day, whoo, I don't think I'm going to be doing that!
    • Various research studies have shown that drivers operating vehicles in cabins that are cooler and less humid tend to be more alert and responsive for longer periods of time, while people in hot, humid, uncomfortable cabins get distracted or sleepy sooner.

    This will not help your car's fuel efficiency.
    (Photo from Polls Boutique)

    Neither will this.

    Sources, Will rolling down windows save fuel or not? August 4, 2005
    Go Hybrid, The Air Conditioning Myth - Does It Really Burn Up Fuel?
    Askville by Amazon, Is it more fuel efficient to drive with your windows open or use the air conditioner?

    1 comment:

    1. I live in Arizona -- suffice it to say that the A/C is on no matter how much gas it burns!


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