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Keeping the Car Cool

The Myth

THIS MYTH WAS FEATURED IN...
Episode Title: Episode 22: Boom-Lift Catapult, Keeping the Car Cool

Original air date: 11/10/2004
Myth Title: Auto Air Conditioning is more fuel efficent


Myth Description: It is more fuel efficient to drive a car with the Air Conditioning on and the windows up than it is to drive with the windows down and the A/C off. This is reputed to be due to the fact that having windows down causes extra aerodynamic drag and causes the car to consume more fuel than that that is consumed by runing the A/C.


MythBusters on the bust: Adam and Jamie


Hypothesis: It is more fuel efficient to drive a car with the Air Conditioning on and the windows up than it is to drive with the windows down and the A/C off.


Procedure/Experimental Design:

Test 1 - Measured Fuel economy
Using a gadget that "monitors air flow through the ... engine" a computer calculates and displays an average fuel consumption for the trip so far and an instanteous fuel comsuption figure. A SUV is driven 5 laps around a race track with the windows up and AC off, 5 laps with the AC off and the windows down and 5 laps with the AC on and with the windows up. The vechicle was driven at 55 mph durring the test.

Test 2a: The average Joe Test

Two identicle SUVs are driven around the race track. One has the windows up and the AC on then other the windows down and the AC off. The vechicles are to be driven until they run our fuel - predicted to take 7 hours.

Test 2b: The average Joe Test short version

The tires would have worn out durring the test 2a. So they have to pump all the fuel from the cars so that they only contain 5 gallons of gas. Other wise the test is as in Test 2a. The cars are driven at 45mph. Jamie drives the AC car Adam the Windows down car.

Results:

Test 1

Windows AC Ave Fuel Consumption (miles per galon)
up off 11.7
down off 11.35
up on 11.7



Test 2b
Jamies car (windows up AC on) runs out of fule 30 laps / 15 miles before Adam's (windows down AC off)



Conclusion: The results of test one are dismissed because they rely on a "computer model" of fuel consumption. The results of test 2b are accepted and they conclude that it is more fuel efficent to drive with the windows down and AC off.


Busted or Not Busted: ---- Busted
Fan Feedback

Highlights of the bust:

  • Jamie's lunch is a brown mush made of blended fruit, grains and vegetables.




Best quotes by the MythBusters:






Your Scientific Method

Did the MythBusters get it right? How would you have busted this myth differently? Share your experiment design for how you would prove/disprove this myth:

  • There is a big problem with the experiment here - it assumes the two SUVs have identically positioned fuel pickup systems, which is unlikely to be the case.
  • A better approach would be to run the two vehicles until fuel stopped getting to the engine (rather than pumping fuel out using an external pump), then adding the same amount of fuel to each vehicle and running them again until they stopped.


  • According to the Automotive Aftermarket Suppliers Association (AASA) "In city driving, you will see a slight fuel economy advantage by opening your windows, but the use of air conditioning at higher speeds can produce a fuel savings," The Auto Channel. So it is possible that the different results obtained from the two different tests could be attributable to the different speeds used in the two test. The Mythbusters results are not definitive.


    • The air conditioning draws essentially constant power from the engine, while the open windows affect the drag coefficient. This means the open windows result in a power consumption proportional to the 3rd power of the speed while the power consumed by the AC is independent of car speed. Considering this, there must be a point (speed) where the two curves intersect and from that speed on, the AC requires less fuel that the open windows. Also, for the test to be fair, both “cooling systems” should be used to keep the same interior temperature (actually, driver's temperatures would be better, but harder). Finally, there are a lot of car dependent factors here like the drag coefficient and engine power and efficiency, and some non car dependent, like exterior temperature (can't keep the car below that by opening the windows).



    See Also

    Related myths: ----

    Related resources and reference pages:

    Use of Air Conditioning in Your Vehicle Saves Fuel Compared to Open Windows, Says AASA-The Auto Channel




    Latest page update: made by ttguy , Feb 16 2008, 12:53 AM EST (about this update About This Update ttguy copy edit section on Drag power cosumption being proportional to speed - ttguy

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    Keyword tags: adam savage byron kari byron kari mythbusters chapman scottie Discovery Channel discovery channel myth busters grant imahara jamie hyneman myth busters mythbuster mythbusters myths urban legends urban myths
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    Threads

    Anonymous  (Get credit for your thread)
    Started By Thread Subject Replies Last Post
    grantpe Keeping the Car Cool myth - even more questions! 4 Sep 30 2008, 6:30 PM EDT by KwikyMAN
    Thread started: Jul 12 2008, 7:46 AM EDT  Watch
    Does which windows are opened affect the results? eg. if only the sun roof is tilted back (more common in Britain where it's not necessarily that hot) than having all the windows wound right down.
    Also, does this follow on to convertible cars, that after a certain speed it's more fuel efficient to have the top up rather than down?
    2  out of 2 found this valuable. Do you?    
    Keyword tags: adam savage Discovery Channel discovery channel myth busters jamie hyneman myth busters mythbuster mythbusters myths urban legends urban myths
    Show Last Reply
    itsbob Keeping the Car Cool Myth.. AC or NO 0 Aug 4 2008, 4:18 PM EDT by itsbob
    Thread started: Aug 4 2008, 4:18 PM EDT  Watch
    Flaw in their testing.

    You can't test a change in aerodynamics using a brick.. no matter what you do to a brick the aerodynamics are going to be BAD. Windows up or down isn't going to make much (if any) of a difference, and no matter what you do AC on is going to be worse.

    Try the AC on or windows down with a economical somewhat aerodynamic vehilce. Something like a Honda Civic, or Toyota Camry, not an SUV!
    0  out of 1 found this valuable. Do you?    
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