During the warmer summer months I get a lot of questions with people asking me if driving with the AC on waste more gas than if it was just turned off and the windows were opened.
Today I want to discuss whether it’s more efficient to keep the windows down and enjoy the warm breeze coming off the road or if the windows should be up and the car AC should be powered on to cool you down.
Personally, if it’s really hot out I prefer having the air conditioning running and during the spring and fall I love having the windows down so I can get some fresh air and it just makes it seem a lot more fun to drive around with the wind blowing through my hair.
Let’s take a look at whether or not you should open the windows or leave the AC running in the summer if you’re worried about overall fuel consumption and efficiency.
Does Driving with the AC on Waste More Gas?
A Quick Look at How your Car Works
Before I get into whether or not it’s a good idea to leave the windows open or have the air conditioning running I want to take a quick look at some simple engine operation regarding your vehicle.
You might not be aware but the battery that’s in your car is actually a rechargeable battery and it’s used to power the majority of the electronics in your car.
You’ll first notice it being used as you turn the key in your ignition to start the engine and at this point the battery is using some of its charge so that it can turn the engine over for the car to start.
The car starter is automated by a small electric motor and that’s why you notice when the battery is dead that the car doesn’t start even though the engine is perfectly fine.
After getting the engine going it’s pretty much self-sustaining as long as you have enough fuel and with the engine running the battery is being charged by the alternator so that it’s ready to go the next time you need to use it.
The Air Conditioner
Since the air conditioner in your car is powered by an electrical system it uses energy that’s supplied to it from the alternator to get everything running.
The energy for charging with alternator is coming directly from the engine and this in turn is using the fuel that you have that’s turned into energy for the engine itself.
You might have noticed that the air conditioning doesn’t work very well if the car isn’t starting and this is actually because the belt that gets the compressor working only works when the engine is on.
So you’ll notice that air is coming through the vents but the compressor isn’t able to compress the coolant so it isn’t able to make the air cold without the engine turned on and running.
It’s the same belt that runs the alternator and also keeps the battery charged while you’re driving around.
Which is More Efficient? AC Running or Windows Down?
Over the years quite a few different tests have been done in regards to the efficiency when it comes to driving with the air conditioning on or the windows down and the results have been pretty concrete in regards to what you should do to keep cool during the warm summer months.
Since the alternator needs energy to charge the battery and run all of the car’s electrical systems it’s obviously using fuel as the engine is running and any extra draw on the alternator is going to have an impact on overall fuel economy.
What this means is that it’s a definite yes running the air conditioning will have an impact on overall mileage and the longer you run the AC the less fuel-efficient your car is going to be.
Many of the different studies have found that running the air conditioner decreases gas mileage anywhere between 5 and 10% and you going to notice this decrease the most during the hottest parts of the summer where the AC is running the most often.
Anyone worried about fuel efficiency or looking to save a little bit of money is going to want to leave their windows down while driving around the city or at slower speeds in rural areas.
With the windows down you’re still able to enjoy a nice breeze despite the warm weather outside and it might be a bit sweatier inside the car but fuel efficiency will be improved compared to if the AC was on and running.
However, keep in mind that at higher speeds there’s a lot of drag that’s put on the car if the windows are left open and this can also have a negative impact on overall fuel economy.
At higher speeds, typically above 45 miles per hour, it’s a good idea to leave the windows rolled up and the AC running, and at lower speeds you can leave the windows open and enjoy the nice breeze.
The best thing you can do is just try to avoid the heat as much as possible and try to get all your driving done early in the morning or late at night when it’s not as hot and you don’t need to run the air-conditioning as high or at all.
The most fuel efficient way to drive around is to keep the air conditioner off and the windows up although for many people this won’t be comfortable and it’s worth considering spending a few extra dollars for some comfort while in the car.