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Does Driving With The AC On Actually Use More Fuel?

Windows AC fuel efficiency

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Does Driving With The AC On Actually Use More Fuel?

We’ve had this question asked a few times recently and decided to investigate it. Which method of driving is more efficient; driving with your windows down and AC off or driving with your windows up and AC on? Today we find out.

Simple Engine Operation

The battery in your car is a rechargeable battery. When you turn the key in your car to start the engine, the battery uses some of its charge to turn the engine over. It’s very similar to a pull starter on a lawnmower, the only difference is, a car’s starter is automated by an electric motor. In fact, some of the first cars had something much like a lawnmower’s pull starter – a crank in the front that one would turn that help get things going.engine operation

Once the engine starts, it’s self-sustaining (as long as there is fuel) thanks to the combustion taking place inside the engine. While the engine is running, the alternator in the car (a small electrical generator) will recharge the battery using some of the power produced by the engine.

If your battery dies thanks to leaving your lights on all night, the electronic motor in the car will not be able to turn the car over and it won’t start up. However, if you were to roll the car down a hill while it’s in gear, the wheels turning will force the pistons to move and the car will start. Once started, the alternator will be able to recharge the dead battery.

Air-Conditioner Operation

When you turn the AC on in your car, it uses energy supplied to it by the alternator. This energy is coming from the engine, which is using the fuel in your gas tank. The AC won’t run properly without the car started because the belt that engages the compressor (which is used to compress the coolant and make it cold) will only run with the engine started. This is the same belt that is responsible for running the alternator and charging your battery.

Which Is More Efficient, Windows Down Or A/C On?

Testing the effect of both methods (windows down, AC off versus windows up and AC on) has been done and debated by several different organizations. One of the leading groups, the Society of Automotive Engineers, conducted a number of tests on the effects of AC versus windows rolled down on automobiles.night driving

The SAE found that running an air conditioner in an automobile decreased gas mileage by 5 to 10%. Stanford University and SAE both recommend driving with your car’s windows down if you are traveling on city streets. However, when on a freeway or highway, the drag produced by your windows being rolled down exceeds the efficiency drop by keeping windows rolled up and AC on. Both organizations recommend that you use your car’s AC when driving at speeds above 45 MPH.

With that in mind, it’s probably best to do your driving in the early morning or later at night when possible. This is when the day is (usually) cooler. Driving with the A/C off and windows up is the most fuel efficient way to operate your automobile.



  1. Fonz

    January 19, 2012 at 12:26 am

    If you’re really worried about fuel efficiency then work on your braking and accelerating. That’s where most of your gas is spent on.

  2. Patrick

    February 16, 2012 at 11:43 am

    I think you missed that biggest draw on fuel efficiency, the AC compressor. The compressor should only run if you’re cooling your vehicle. This is the largest draw fuel efficiency from AC. If you’re sitting idle, such as at a stop light, you’ll notice your AC compressor turn on/off as evident by the engine RPM dropping/increasing respectively. Any thing that causes the engine RPM to change noticeably is certainly going to impact fuel efficiency.

  3. Steve

    March 22, 2013 at 5:44 pm

    The compressor is a load on the engine. The load requires energy for the work. Turning the AC on increases the load thereby increasing the amount of energy needed. The engine works harder burning more fuel.

    • Matt

      February 1, 2014 at 12:24 pm

      Exactly right. They went into a bunch of detail about the battery, etc., but the battery has nothing to do with it. The compressor is engaged through the same belt system as everything else. If the compressor turns on (cooling), then there is a bigger drag on the system overall, reducing it’s efficiency. I always think of it as rolling down a hill in neutral… pretty easy. If you are on a steep hill though, and put your car into first gear… will it still roll? Probably, but not as easily.

      • Marvin

        September 12, 2014 at 10:46 pm

        Technically you cannot say the battery has ‘nothing’ to do with it. Your A/C system utilizes and electric fan to move the air in the cabin. Anytime you use electricity it causes your alternator to work harder which adds ‘drag on the system overall’, as you put it. So, though it is ever so minimal, the battery does have something to do with it.

    • Rick

      July 26, 2019 at 7:49 pm

      You are correct, however, the ac compressor doesnt draw away a significant amount of horsepower from the engine. You may lose a mile per gallon but I think that being comfortable and not stuck to your seats more than compensates for that loss.

  4. Zubair

    October 26, 2013 at 9:17 am

    With AC off, my car’s average is 170 km and with the AC on the average is 140 km with a full tank of 8kg.

  5. Doug

    July 20, 2014 at 8:36 am

    If you have a look under the hood while the engine is running and the AC is on, you will notice and hear the compressor kicking on and off. This is activated using a clutch attached to the fan belt, it cycles on and off to maintain the level of coolness of the interior of the car/truck.

    Here’s another tidbit: most people don’t realize the windshield defroster is actually like running the a/c, which directs heat to clear the windshield of any fog or ice/snow.
    After the windshield is clear, it would be wise to switch off the defroster and send the heat to the floor vents.

    • Marvin

      September 12, 2014 at 10:42 pm

      In most newer cars (meaning anything less than about 30 years old) the defroster function uses a mixture of hot air from your heater system and cold air from your air conditioning system. The reasoning is that you don’t want to shock an ice covered windshield with air that can be well in excess of 140 degrees. Most engines have a water thermostat that ranges from 180 to 195 degrees. If you warmed your car up to operating temperature and then turned the defroster on to a frozen windshield, you could heat it too quickly and possibly crack your windshield. Mixing the cold A/C air with the heater air in your defrost system cools it to a safer temperature preventing damage too your windshield while still offering hot enough air to defrost/deice the windshield.

      • Austin

        September 16, 2014 at 4:11 pm

        The defrost setting utilizes the ac system not to reduce thermal shock, but because the evaporator inside the car withdraws a significant amount of moisture from the air and the windshield is cleared much faster with dry air. Moisture extraction is why there is a drainage built into the evaporator housing, and it’s also why you always see water dripping from a car that is running the AC.

  6. Wade Hawk

    August 14, 2014 at 2:12 am

    However, if you were to roll the car down a hill while it’s in gear, the wheels turning will force the pistons to move and the car will start.

    This won’t work on a lot of modern cars with electronic ignition and a computer, but works on any car with points and plugs. If battery is fully dead you are SOL trying this method on most modern cars. If you want to know if it will work on your car, park on a hill and unhook the battery. If it works on your car you shouldn’t need the battery, then try and see if you can roll it off.

    I found this out the hard way while trying to roll off a 2002 Mitsubishi Mirage with a completely dead battery.

    • Marvin

      September 12, 2014 at 10:31 pm

      A car having points and plugs has nothing to do with it. It is all about having a generator or an alternator. If you unhook your battery, your car will only start if it is equipped with a generator and not an alternator. A generator works with permanent magnets and a coil to produce electricity, an alternator uses electromagnets and a coil to produce electricity. The use of electromagnets requires a small amount of voltage to be applied to function. A completely dead battery or an unhooked battery will not allow the alternator to function. A weak battery that lacks the power to turn your starter motor might still have enough voltage to allow the alternator to function and produce electricity. Also, all gasoline automobile engines have spark plugs. They either have a pointed distributor or electronic ignition, but they all have spark plugs.

  7. mikey

    December 16, 2014 at 10:05 pm

    First of all, don’t ever try push-starting an automatic. Manual transmission only. Secondly, what Austin said. Hes right!

  8. Shane Norris

    December 28, 2015 at 6:47 pm

    Actually while the alternator spins the same speed relative to the engine all the time, the load it places on the engine varies depending on electricity requirements. The alternator uses an electromagnet instead of permanent magnet to interact with the windings – by changing the power through the electromagnet the field strength and therefore the force the engine needs to drag the windings through that field increases (along with the amount of power the alternator generates).

  9. Dan

    June 15, 2017 at 4:31 pm

    What I’d like to know, is if there is a gas higher gas drain if you use the internal air. I think they used to call this Max. I have a 2009 Saturn Outlook(Same as Arcadia) and can’t decide if this uses more fuel. Logic would suggest you run air from the outside in the beginning since the internal air is most likely hotter, then switch over to internal when it cools. But does this save on gas over regular AC, or increase because your asking for more power? Thanx in advance!

    • Rick

      July 26, 2019 at 7:47 pm

      Using the AC on Max setting is more efficient as it recirculates much of the air inside the vehicle instead of trying to constantly cool hot air from the outside. You are NOT asking for more power when you switch to Max. The only thing that happens is that a damper closes and your fan speed may increase a little, however, electrical load doesnt affect a cars gas mileage. Your alternator turns as the same speed all the time be it winter or summer. An alternator doesnt produce more drag when the electrical load increases as that load is taken up by the battery. An alternator does not supply electricity to your car, it makes sure you battery stays charged. This is why you can still drive a car for short time even if the alternator goes down.. the car will still draw electricity from the battery.

  10. Rick

    July 26, 2019 at 7:44 pm

    Your alternator doesnt draw more horsepower from the engine when the AC is on. It is not like it drags more as it turns. The only thing that draws horsepower from your engine is the AC compressor and even that dosent draw that much. Car AC system, unlike home AC systems, use a very low pressure refrigerant and it does not take much power to compress it to a useable temperature. Also, once a comfortable temperature is reached within your car, you can turn the blower down and your AC will cycle less and less. Absolute myth that running the AC causes a significant loss of gas mileage. Driving with windows down causes wind drag and that reduces gas mileage. Your choices are, lose mileage while driving with the windows down and in a 90 degree sweatbox or lose gas mileage while you are comfortably inside a 75 degree car with low humidity. Your choice.

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