Surprise: Using car's air conditioning saves gas

Discussion in 'Fuel Economy' started by JohnM, May 25, 2015.

  1. JohnM

    JohnM Well-Known Member

  2. 08EscapeHybrid

    08EscapeHybrid Moderator

    Yeah, at highway speed, the aero drag from riding with the windows down is higher than the drag caused by the a/c compressor. THis may be news to the average motorist, but not to most folks here. If I'm going w/o a/c at highway speeds I keep the windows up, and run the vent fan on high.
  3. RedylC94

    RedylC94 Well-Known Member

    Yeah, sure, but only at high speeds, only if all the windows are all the way down---in tests originally performed nearly 40 years ago.
  4. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    I'm not a big fan of AC. You may come out ahead a bit at higher speeds, but I doubt by much. You can too, just crack the windows, say an inch, on the highway.

    Watching the fuel consumption per trip with a ScanGuage, I find the liters per 100 km stays stubbornly higher if I'm using AC.
  5. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    :rolleyes: Just more justification for doing nothing and just accepting the world as it is. Also poorly researched and presented. Luxury has become the new necessity, completely lacking in perspective.

    As others have said, rolling windows down just a couple inches is plenty of ventilation (for me ;)) for anything below 100F. It adds almost NO aero drag, especially on modern sleek vehicles.
  6. seftonm

    seftonm Veteran Staff Member

    I'll fire up A/C above 50mph, especially if I have others in the vehicle. Makes it more comfortable and easier to have a conversation.
  7. SpartyBrutus

    SpartyBrutus Hypofueller

    I tend to do this if starting out on a hot drive:

    1. Remove front full windshield sun visor.
    2. Open windows while driving side street(s) for a few minutes to let the car air out and cool down.
    3. Close windows and run AC in "recirc" mode either at a low fan speed and/or higher auto temp control. I assume I'm not "cooling the whole neighborhood" this way.
    4. Every so often, switch to "once through" AC mode.

    This might cost me an additional about $0.07 on my daily 38mi drive.
  8. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I override the car's automatic "recirc" ac on hot startup. Outside air at 100F is cooler than inside the car at 130F (measured! :eek:), so the AC can work faster from a lower starting point. Once the inside air (and materials) are cooler than outside, THEN I switch it to recirculate.
  9. ALS

    ALS Super Moderator Staff Member

    I'm lucky my Prius, the AC unit reads interior temps as two or three degrees higher than they really are. I can get away with setting the AC at 75-76 and I'm very comfortable on the highway. I wouldn't want to see the drop fuel mileage if I had it set at 70 degrees. :rolleyes:
    Plus with it being painted white makes a big difference when driving in the summer heat.

    The front windshield sun shield cuts the interior temperatures by a good ten to fifteen degrees when parked. I highly recommend getting one for your car to save your dash and lower the interior temperatures when your parked.
  10. fuzzy

    fuzzy Mild hypermiler

    Both my original Prius (2010) and current upgraded one (2012) are silver. But the current one has the solar sunroof option, which changes the roof color to black. It certainly seems like I feel the difference, this black-topped one is hotter while driving out in full blazing sun.

    But parked is a different matter. Despite the black roof, that continuous solar-powered ventilation does keep it cooler, making for more comfortable returns to the car.
  11. ALS

    ALS Super Moderator Staff Member

    I know how you feel my Volvo is black and I've driven through southern Georgia in 100 plus degree heat which actually topped out at 106 and it was everything the car could do to cool the interior. I was so worried, I dropped down from 65 mph to 60 mph to take some strain off the engine and cooling system. Normally the Scan Gauge would be showing 26-27 mpg and with the extreme heat I was lucky to see 20-21 mpg.
  12. aca2983

    aca2983 Well-Known Member

    We're not into full summer heat mode yet, but I've had a 15% tint installed for one week and can tell a difference in comfort. And there are times when I am not running A/C when I otherwise probably would have.
  13. some_other_dave

    some_other_dave Well-Known Member

    Additionally, I believe that the trade-off also depends on a lot of other factors, like how big your engine is, how aerodynamic it is, etc.

    I know that a 60s-vintage AC compressor on a small 60s four-banger has a really immense impact on the economy, while a modern rotary compressor on a 5-liter V8 has pretty little. And cars with really horrible aero probably don't get affected as much by open windows, while cars that are very slick with the windows up might be affected significantly more (relatively) with their windows all down.

    To make definitive conclusions, they have to make dozens of simplifying assumptions--to the point where their conclusion is just about worthless, IMHO.

  14. jberman

    jberman Member

    I don't really notice much of a difference from running my AC to even against my windows up with my real-time mpg calculations on my 15 Corolla. I typically set my automatic climate control to 75 (almost always recirc unless my interior is hotter than outside), and I still manage to get 50+ mpg on my trips, in a car with combined EPA rating of 32.
  15. RedylC94

    RedylC94 Well-Known Member

    Size of the engine should have no effect on the extent to which air conditioning increases fuel consumption per mile or per hour, assuming the engines are equally efficient. Obviously a high-mpg car will lose more miles per gallon than a low-mpg vehicle, if the increase in consumption is equal.
  16. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    I installed a Pass Master device on a few of our Accords, over the decades. It's a vacuum activated switch, closes the circuit as long as there is high vacuum, breaks the circuit if vacuum drops. You splice it into the AC compressor power lead, and hook up it's vacuum hose to the intake manifold.

    Under heavy acceleration, vacuum drops, and the AC compressor is momentarily disabled. It's a simple, effective device. It's only really noticeable if you're running up a sustained uphill grade.


    Last edited: May 28, 2015
  17. 08EscapeHybrid

    08EscapeHybrid Moderator

    In the 80's K Cars did that. I hated driving my mom's LeBaron in the mountains because the a/c would cut off on the climb.
  18. NeilBlanchard

    NeilBlanchard Well-Known Member

    I concur with Andrew: cracking the windows (up to ~1 1/2") I have found improves the aero drag a bit over having them closed.

    So, running A/C is always going to be worse than opening the windows slightly.

    I only run the A/C if it is just way too hot, and I take the hit to my efficiency.
  19. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Lowering probably offsets the window swamp cooler.
  20. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I thought that was a JATO unit to assist in acceleration. Those cars were mighty slow.

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