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.
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.
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?
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.
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.