You might remember as a kid trying to turn AC on in your parents’ car and getting the scolding of your life. “Do you have a secret stash of fuel somewhere that we don’t know about,” came the rebuke. It was ingrained in our minds from an early age, you don’t use AC, you roll down the windows.
With summer upon us, you might want to know just how good that advice is. Turns out, that age old wisdom is solid. It really is more fuel efficient to roll down your windows than it is to use the aircon. However, that’s not where it ends. There are nuances.
The Society of Automotive Engineers (SAE) conducted a study a while back that showed that it is more fuel efficient to roll down your windows up to around 70km/hr. Of course it will be different for different cars but as a rule of thumb, 70km/hr is the golden figure.
When you exceed that speed, the drag caused by the open windows will increase fuel consumption more than turning on AC would.
Using the aircon can increase fuel consumption by 10%. That’s a lot. However, rolling down the windows at higher speeds can increase consumption by up to 20%.
To note is that this applies more to sedans than to SUVs and other aerodynamically challenged vehicles. Turns out SUVs already have so much drag that rolling down windows at high speeds doesn’t do much. In some studies, rolling down windows was more efficient than using the aircon in SUVs even at high speeds.
So, generally put, city driving is windows down but highway driving is aircon territory. You will find studies that say windows down is better at any speed for any type of vehicle so just know the debate is far from settled.
The fan
There is another option. The fan on but AC off option. This won’t result in any significant increase in fuel consumption because the fans will be run off the batteries just like the stereo. The aircon involves running a compressor off the engine hence why it increases fuel consumption.
This method won’t cool you as well as rolling your windows down will. The small fans can’t compete with the volume of air that rushes in from the windows. However, the fans may be the only option when the air outside is polluted and you’re in stop-go traffic.
In that scenario, the humble fan is your best option. You just need to make sure you set your dials to recirculate cabin air and not allow air from outside the car. The effectiveness of the fans will depend on how hot it is, you must know how fans can be ineffective when they are pushing hot air around.
Heating the car is easier though as the engine produces a lot of heat and that can be channeled to the cabin. Heating is as good as free.
So, on those rainy days when it gets a little chilly, you don’t have to think twice about turning the heat on. The fans will have to push the hot air in but we have already established that fans are efficient.
If you have a newer car it’s not as clear cut. You may not have the ability to manually turn fans on with AC off.
Live like a king/queen
There you have it. Don’t sweat like a pig, drive in comfort without breaking the bank. Heating is practically free as are fans with AC off.
As regards the other thing that would get you a good spanking – turning the cabin light on at night – the parents might not have been that spot on. You have no doubt heard that the cabin light should be off when you’re driving at night.
The logic is, it is hard to see into the darkness when you yourself are in the light. Try peeking outside your window at night with the light on in your room and see how difficult it is. However, if you point a light into the darkness, it doesn’t matter much that you are in a well lit room. You can see well enough.
This means the cabin light issue is simple, unless the light distracts the driver there’s nothing to worry about. Except maybe privacy. In some cars the position of the light, coupled with the slant of the windscreen may result in some glare in the driver’s eyes.
Clearly, in that case it would be unwise to turn it on. If not though, you can have your passengers read books from the cabin light, it’s okay.
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