A/C or Windows Down: Which is Better for Fuel Efficiency? February 17, 2014

A/C vs windows down One of the most hotly debated topics when it comes to cars and trucks is definitely the problem of air conditioning versus keeping the windows down and which is more fuel-efficient. The window-down crowd believes that A/C draws too much from the engine, which hurts efficiency while the A/C bunch believe that keeping the windows down hurts aerodynamics, causing worse fuel efficiency. So the question is, who’s right and is it OK to blast that A/C all summer?

The Case for Windows Down

ac-windows-down-window-better   (Photo courtesy of Independent Expression Photography) As any dad will tell you on a road trip, running the A/C kills fuel economy. If you listen to the dads of the world A/C will make your truck or car’s fuel gauge visibly move. A/C hurts fuel economy due to the compressor that’s needed to operate it. The compressor is parasitic, taking power from the engine itself via a belt, which makes the engine have to work a little bit harder. It’s this parasitic power loss that hurts your MPG, so to stop this loss; many believe that putting windows down to keep cool is the better option. While A/C does feel better than air blasting into your truck, at lower speeds windows down is actually the better option. When talking about a truck that is inherently not very fuel-efficient to begin with, the additional drag caused by windows down at speeds below 55 MPH is negligible at best, so if you care about fuel efficiency, windows down is definitely best. To better prove this point, MythBusters took this myth on and tested it for themselves. The hosts of the Discovery Channel show placed 5 gallons of gas into two identical SUVs and took to the Altamont Raceway at a set pace of 45 MPH. One had the A/C going and the other had the windows down. When it was all said and done, the SUV with the windows down was able to get 15 more miles out of the 5-gallon fill.

The Case for Air Conditioning

ac-windows-down-air-conditioning (Photo courtesy of flickr) While windows down is great at 45 MPH, the idea is that the windows being down causes a loss of aerodynamics, which is one of the biggest draws on fuel economy. This is why people follow closely behind semi-trucks, so they can take advantage of the truck’s split in the air. As speed increases past 45 MPH however, the fuel economy loss that windows down causes will increase while the loss from A/C will remain the same. Since A/C uses the same amount of parasitic power loss no matter the speed, it’s loss is a straight line on a graph while windows down is a curve. At a point the two will meet and then cross, making windows down the biggest loser. This crossing point will be different for just about every type of car and truck out there, so determining it is very difficult. For example, a sports car relies on aerodynamics to go faster, so fuel economy takes a much larger hit on them than a big truck. This means that the line where A/C loses less fuel efficiency is at a much lower speed.

So What’s Better?

ac-windows-down-better   (Photo courtesy of flickr) This battle is about drag versus parasitic power loss. Drag increases with speed, so the faster you go the more windows down will affect you, while A/C’s impact will remain the same. This means that at lower speeds windows down are definitely the better option and can save you possibly 15 miles per tank, while higher speeds are best for A/C. This actually works in practice, too, as nobody likes to have his or her windows down while driving at 75 MPH, but it’s nice to cruise with windows down when you’re going 35-45 MPH. So which one is the winner? Well, that all depends on your car or truck and what speeds you drive at. HowStuffWorks.com explains this in far more detail, and even links to the SAE test that proves this idea, too. While your father will probably never agree with this, A/C is better when you’re going faster, and windows down is better when you’re going slower. Now you know, just don’t try to correct dad.


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