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.
Does Driving With The AC On Actually Use More Fuel?
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, 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.
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.