Common A/C and Fuel Efficiency Myths Debunked


Two A/C myths always seem to crop up as the temperature climbs higher and long road trips become commonplace.

Myth #1: A vehicle’s air conditioner causes the engine to work harder. Therefore, electing not to use the air conditioner and instead rolling down the windows when driving will significantly increase fuel mileage.

Myth #2: Driving with your windows down will significantly decrease your fuel mileage because of the increased aerodynamic drag the open windows create.

One myth probably has some truth to it and one is most likely false. Here’s why.

Car_air conditioner

Uncovering the truth

In a test conducted by Consumer Reports, they drove a Honda Accord at 65 mph and found that using the air conditioner reduced fuel mileage by 3% (versus keeping it off). In another test they drove at 65 mph, but this time with the windows down. They found no measurable effect on fuel mileage.

In a similar test performed by Edmunds using a Toyota Tundra, they saw a decrease in fuel mileage of almost 10% when using the air conditioner as opposed to driving with the windows down and the air conditioner off.

There are many similar tests and results online, but here’s the bottom line:

  • Using a vehicle’s air conditioner may result in a small decrease in fuel mileage. As the driver, you get to choose what to do with that knowledge. Maybe that decrease is negligible compared to the discomfort of not having air conditioning on a hot summer day.
  • Driving with a vehicle’s windows rolled down doesn’t produce any measurable impact on fuel mileage as a result of aerodynamic drag (but your dog will love it if he’s along for the ride).

How to really improve fuel economy

If you want to improve gas mileage, try some of these fuel-saving strategies instead:

  • Slow down and avoid aggressive driving, such as hard accelerations and hard braking. Driving normally will increase fuel mileage by as much as 33% at highway speeds.
  • Remove excess weight from the vehicle and avoid hauling bulky items on the roof because it increases aerodynamic drag.
  • Keep your engine in tune and tires inflated to the recommended air pressure for a 3-4% improvement in fuel mileage.
  • Consolidate trips: do all your errands in one run

What do you think? Do you prefer to sweat if it saves you a few pennies, or is it a small price to pay for personal comfort? Leave us a comment.


  1. checker99 says:

    The smaller the engine the bigger hit A/C makes on mpg. For my car its 12% or 6mpg on the highway.

  2. Mike L. says:

    Best Tip for Greater Fuel Efficiency: Don’t buy an SUV or Pickup in the first place since 90% of us don’t haul anything more than what a 4 cylinder hatchback can handle.

  3. The effect of using AC on gas mileage really depends on the car and the efficiency of the compressor.
    In my 2003 Mazda Protege5 using the AC has about 10-15 percent reduced MPG. While my CX-5 has maybe a 1-2 percent reduction. My 1991 Toyota MR-2 Turbo has about 5-10 percent, although I do not drive it enough with the AC on to give an accurate difference.
    The AC compressor in the Protege5 is not as effient as the others. Also, turning off the AC during acceleration feels like giving the car a shot of nitrous.

  4. your condenser coil located on the front of the radiator should be cleaned by spraying it off to remove bugs and other highway dirt. do this in the early spring. best done at the car wash. use a foaming cleaner on it. this will increase a/c performance greatly. be sure and use a non acid cleaner on the condenser coil because it is made from aluminum.

  5. How about the myth ( actually a fact that running your airconditioner constantly makes it last longer?
    I have proven this with three Toyotas a Chevy and a Pontiac. Where by 75,000 miles parts need replaced on the average summer use only a/c. I have never needed to recharge or repair the a/c’s on these “constantly run” vehicles. One at 148,000 miles a 2002 Toyota Highlander. The Chevy Impala was 12 years old and 110,000 when I sold it the Pontiac Transsport at 178,000 miles a 2004 Tundra at 68,000 miles.
    Now deciding whether the cost of paying out X dollars over a lifetime of a vehicle or having to come up with $400 to $1200 at one time to repair s system is the decision you have to make.

  6. I agree with your findings. In my humble opinion your fuel economy suffers even more driving in town with the AC on. I to like the feel of fresh air and drive with the windows down every chance I get, even in town. I also agree, I will suffer the decrease of fuel economy to have the AC running on a really hot and windy day.

  7. This was an interesting article with some of its own questionable “myths.” For example, hard stopping causes an increase in fuel consumption? In an automatic tranny I just can’t see that. Sure in manual you may downshift, but if you release the gas pedal you reduce gas consumption. I am sure it is pretty minor anyway.

    One issue I find seriously lacking in our vehicles is a graph that computes fuel consumption based on speed. In every aircraft I have flown there is a chart that shows fuel consumption based on speed. The chart showed that up to a certain speed you increased fuel consumption until you reached a certain speed, such as Long Range Cruise, which was the most optimum speed for fuel. Above it or below it and you used MORE fuel.

    So far I have figured out that my little Chevy Sonic burns less fuel at 65 vice 55. I just wish I had a chart to give me a better starting point.

    • This is a simple one. Each and every time you apply the brakes you dissipate the energy it took to get you rolling and up to speed. You then have to apply the throttle and get back up to road speed again. Its what comes after the braking that lowers your mileage. Cruising on level road is the most efficient of course. Accelerating (especially from a dead stop) is the least efficient. A good example is on a four-lane highway coming up behind a small line of vehicles slowing to exit. If the left lane is free and clear, its better to safely ease over into the left lane without slowing down than it is to stay behind the line of exiting vehicles and reducing your speed. This lets you avoid the loss associated with getting back up to speed (since you didn’t have to show down).

  8. Ed Schwerin says:

    Using the AC in hot summer can sometimes cause overheating. If you find the car overheating while idling or at stop lights or in slow traffic check the fan clutch. If the car is overheating in general, check the coolant level and/or radiator cap. A general cooling system check of the radiator, water pump, and heater core, may turn up leaks. Advance Auto carries all of these necessary parts to combat hot weather driving problems.

  9. Michael McLeod says:

    There is another myth about A/C: Don’t turn your A/C on at highway speeds, as this will stress the compressor going from 0 ROM to say 2500 ROM in an instant. But many vehicles cycle the compressor so the saving of one of these is a waste of time and makes no difference.

  10. Dale Determan says:

    On a hot summer day when I’m on a back road, I like windows down & air on to. You know just so I can see out better,and keep cool to.

  11. Lee Young says:

    my late mechanically-professioned landlord told me years ago you better run your a/c at least a few minutes EVERY month just to keep the internal rubberized parts ‘lubricated’

  12. i drive in town with window down and a/c on.

  13. Oh yea. Sure I’ll just drive a fuel efficient car. What isle do I find one of those on. I was in your store earlier today and didn’t see any, perhaps you guys were out of stock ???

  14. Ted brooks says:

    You are incorrect. Oxygenated gasoline will vary in the amount dispensed with ambient temperature by up to 5%. Cold temperatures will always give you more for your money as the dispensing pumps do not calculate the temperature when measuring. Check with the bureau of weights and measures for accurate information on this subject

  15. what about the myth of needing a little refrigerant? air conditioners take a very specific amount of refrigerant. they are hermetically sealed. meaning they should never leak. if they leak something is wrong. topping it off is not the recommended procedure. that’s precisely how we got into the mess we did with freon 12. although it’s become easier and less damaging to the environment the new refrigerants I’m sure cause damage somewhere else so constantly releasing them into the atmosphere from a poorly maintained system is not a good idea furthermore it’s illegal. if your air conditioner is not working and you see oily deposits around the fittings then you have a leak. topping it off will only be a temporary fix and anything you put in will eventually come out. service it properly or take it to a professional. a lot of times it’s only a couple of o-rings but without the proper equipment you may never get it fully charged.

  16. George Burke says:

    I enjoyed reading your comments, and for the most part, I think that you are 100% correct. However, about buying gas in the morning. I worked in a convenience store that sold gas from 1972 thru 1977. We were told that the gas was delivered from the tank farm at 60 some degrees. The only thing that I know is that we dipped, (measured), all three tanks daily. We always lost gas in the winter, and always gained in the summer. Year round for us, it probably evened out. So there may be something to buying in the morning, especially in the summer, and if you had a 100 gallon tank, like some trucks. We did not have diesel, so I have no knowledge of the properties of that product. Best regards, G. Burke

  17. Kurt Billmann says:

    I have found that driving a pick-up truck with the tailgate down will reduce your fuel milage. Reason is: as the air flows over the cab & into the bed of the truck it needs the tailgate to abate that flow & keep a positive pressure ridge so that the air flows smoothly over the cab. With it down, now the air is turbulent & you use more horsepower to over come that.

    Also I have gotten the best gas mileage on a epic road trip by driving at a constant 66 MPH on cruise control with the air conditioner on & windows rolled up in my 2005 Dodge Ram 1500 Quad Cab 4X4 & that was 18.86 MPG the sticker said 17 Highway.

    I also found that if you are driving at less than 41 MPH in the city use your “Tow/Haul” button to prevent it from shifting into direct lock-up & back I get a 1-2 MPG increase in gas mileage doing that, plus it saves on the transmission wear & helps slow you down when coasting to a red light.

  18. Dave Ritchie says:

    Ok, I like that you’ve debunked some of the a/c myths, but driving with your windows down — allowing your dog to stick his head out into the wind — can seriously affect his vision now or later in life. Take a look at:

    You might edit out your off-hand comment about the dog, because it defeats your purpose when you’re trying to debunk myths.

  19. Two vehicle model tests does not make for a valid conclusion, and especially not if the test is while driving a continuous 65 MPH which will offer lower fuel economy in general than driving at 55MPH, but also cause the A/C to be a much smaller % of total load than stop and go driving or driving in a lower gear, below ~ 50MPH. It also doesn’t take into account that a vehicle is new only once, that if your A/C efficiency goes down as it ages and/or leaks, it has to run longer to maintain any particular temperature drop.

    The only way to know for sure is to test your own vehicle both ways within the typical driving environment you face most of the time, remembering that you need your vehicle fully warmed up prior to either test so the engine computer is operating in closed loop mode to maximize fuel economy.

  20. I believe that a couple of things in the automotive arena have been improved to a point where the old sage advice no longer applies. If we think back to the time when the advice was first offered, cars were weighing in at an average of 2 tons, and boxy is in no way aerodynamic, and AC compressors were heavy and required about 10HP to operate. Reduced vehicle weight, better aerodynamics and lighter more efficient compressors all add up to fuel efficiency at its current best.

    My Olds 422, 455CID engine and A-6 compressor averaged around 14MPG, air on or off didn’t matter much, If I floored it once, and the back barrels of the quadra-jet 4BBL carb opened up, mileage dropped off to about 10MPG, do it twice, you were down to 8. I swear I could see the fuel gauge drop lower while my foot was in it.

    I don’t mind heat nearly as much as I do humidity. AC’s ability to remove moisture from the air is worth every cent it costs to run it. I am happy with the fuel economy I receive, and if it fails to deliver on it, I look to see what maintenance is required in the owner’s manual for it! Don’t overlook cleaning the fuel injectors and throttle body area, deposit resistant is just another term needing to be eliminated from the books.

  21. What about the myth that you should shut off the A/C and let it decompress just before you turn the ignition off? The belief is that it is easier on the seals, and its also better when you start the car to have the AC off so you have less stress when your cranking the starter.

    • On some cars, the sweat on the evaporator sticks around when you turn off the car, and the moisture gets a mold/mildew smell. So, if you turn off the A/C and let the vent run a minute before killing the ignition, the evaporator will warm up and the blower will dry it off.
      As far as the stress on the starter.. When the compressor stops turning, the system begins to equalize pressure right away. If it’s an orifice tube system, this happens quickly enough that you can hear it hissing. On some TXV systems, it takes a little longer

  22. how much power does an a.c system of a ford ranger consume?

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