Air Conditioners Save Fuel?

No they don't. Here is another debate pitting science and reason against vested interests and wishful thinking. The science is simple enough for me to understand, and can be proved wrong without an advanced engineering degree. The debate is about using air conditioners in cars to save fuel. If you are not aware of this debate, just keep an eye out this summer, when it starts to appear on TV newscasts, and in newspapers and various other places. In every case, the fact is stated that rolling down the windows wastes more gas than turning on the air conditioner. The "expert" advice is "turn on the air conditioning to save gas", nobody speaks up for the other side.

Here is a link to one of many debates.


I love the AC debate, because it is actually possible to understand the science in it without dumbing it down. Unlike the Global Warming debate. I have not been able to nail down any sponsors of the AC propaganda, so it may be spontaneously generated because the motoring population simply wants to believe it.

The point of the argument is to justify the use of air conditioners in cars even in times of high gas prices. So the idea is put forward that cars use less gas with the windows up and air conditioner on than they do the other way round. Reports to this effect often appear on TV news, delivered as simple truth with no counter argument.

Unfortunately most people don't measure their gas mileage accurately enough to know the difference. That takes a fairly disciplined and rigorous test lab such as Consumer Reports. It's pretty obvious using scientific theory that even running the ventilator fan or the dome light will use more gas, but is it large enough to be measured when you take into account all the variables?

Look at the side of a car and notice there are two round openings on each side, each could be equivalent in wind drag to an open window. They are the wheel cutouts (around the tires). Back in the fifties, car makers experimented with covering the back ones for streamlining, but the gas it saved was so minuscule that it was not worth the effort, and the idea almost disappeared.

The fuel efficiency test methods I thought were most accurate were by Consumer Reports in 2005, where they found a 1 mpg difference in fuel mileage with air conditioner on or turned off, and no measurable difference windows open or closed. The 1 mpg, I assume would be greater if you had a car with no air conditioner installed - no drive belts to the engine, no weight or maintenance issues. And of course, eventually no wasting gas driving the car around to get the AC fixed. You may even be be tempted to drive a car a bit less if it had no air conditioning. Also you save money in the cost of the car.

Cars are now being designed to need air conditioning. They have inadequate air circulation, and sloped windshields that trap heat like a greenhouse. Cars of the fifties sometimes had peaks over the windshields, and all had really effective air circulation.

I was also wondering what goes on with race cars. The beauty of racing is that it tends to put a stop to a lot of this type of scientific distortion. But my research was inconclusive, as NASCAR racers often run with the side window open and with an AC unit on. I guess it gets really hot in there.

At least motorcycles don't need air conditioners yet, I'm guessing it's coming.