Annotated Mythbusters

Windows down vs air conditioning.
I believe that the conclusion "myth busted" is perhaps a bit hasty. The urban puzzle is based on the fact that the car generates more drag with the windows down. This is of course the case. However, the speed of the car should be taken into consideration. As one knows the wind resistance increases with the square of the speed. Since the test was done at relatively low speed it does not automatically mean that the "myth is busted". At higher speeds, the effect of the windows open on the fuel consumption also increase with the square of the speed. This is however not the case with the AC on and the windows closed.
I believe a revisit may be in order.

Good luck! Great program!

The second test with two different vehicles was not much of a test. You simply cannot assume that both vehicles will get the same fuel mileage while carrying the same weight driven and driven at the same speed. One will get better gas mileage. That is a fact. The test was done with a flawed assumption. The only true test would be to test the SAME vehicle with the A/C on and then again with the A/C off. I am surprised and disappointed in their oversight. (undersight) Always a pleasure to watch though.

The a/c test was interesting but the bigger myth is how 2 brand new Ford SUV's can get 11.7 mpg at highway speeds when they are rated at 18 mpg highway by the EPA. Even more worrisome is that they couldn't safely be driven at highway speeds on a closed track with slightly banked gentle turns.

If the Official Mythbusters folks should see this:

I second Rudy's comments, with additions. First, using a beast such as an SUV means that the A/C or windows can add only a small incidental load to the whopping great engine. You should do this test with a sedan, with a modest motor. This will magnify the effect of both methods, which will allow a more precise measurement of the difference. Let's not chase fuel efficiency from cars not even remotely designed for it.

Second, the aerodynamics of an SUV skew the test. SUVs are far less aerodynamic than some notional "average" car, and therefore will show less effect from the open windows. You should use a car with a lower coefficient of drag. By using an SUV, you have skewed the test results toward indicating that A/C is less efficient.

The short track and low-speed approach mentioned by Rudy are also problems; please take Rudy's suggestions as well as mine.

This is a topic which I think every driver has contemplated, and which would yield another good show, what with gas prices remaining so high.

Whaddya say?

I don't have a dog in this fight, by the way--I ride a motorcycle (in Japan, no less), so I naturally use the "open windows" method, but there are days I would gladly use A/C if it were available!

I have to lean towards the fact that A/C consumes power on it's own. Being a NASCAR mechanic I can honestly say that I am a professional! One of the first things to be gutted from a track car is the A?C unit. Why? Because you LOSE AT LEAST 6 HP of energy from turning the A/C unit to it's lowest setting.

SO, it's common sense if you try it yourself. Driving down the road with windows up and the radio off, try traveling around 60mph and feel what happens when you turn the A/C on. You can feel a definite pull in the engine and a drop in performance with the A/C on versus the windows down and A/C off. I have personally tested engines with the A/C unit intact versus the A/C unit removed on a dynomometer or Dyno. The EXACT same engine showed a 12 HP drop with the A/C unit on.

I know when you take aerodynamics into consideration you lose approx. 8 HP and weight costs you approx. 20 HP on a full size SUV, but even if you take that into consideration for both of the vehicles of the same make, model, and engine size the numbers speak for themselves. An engine consumes more fuel when the A/C unit is switched to the 'On' position. Try it yourself if you don't believe me!

Waltrip_Racing-- I don't think anybody is questioning the fact that the A/C causes a loss in power and thus will cost you gas mileage. The debate is whether or not the loss to do wind resistance (with the windows down) exceeds the loss due to the A/C. As far as I understand, the loss due to A/C should be fairly constant regardless of what speed you are traveling at. Wind resistance, however, increases with speed. For example, stick your hand out the window when you're going 10 mph, then 35 mph, then 70 mph. You'll observe that the faster you are moving, the greater the force that the wind exerts on your hand is. This is same force (wind resistance) is also pushing back on your vehicle.

So at small speeds the effect is negligible, and of course running the A/C will be worse than having the windows down. But as you get faster and faster, the resistance will increase, and will cause a greater and greater loss in mileage. At some point, unless the resistance force reaches an asymptote (which I doubt it does), the wind loss will eventually exceed the loss due to the A/C.

Therefore, the question becomes, "above what speed is it more profitable just to put your windows up and turn on the A/C", as opposed to "which gives you better fuel mileage".

Now, once you understand the question, it should help you in understanding the comments given above. Those that posted before you basically covered the biggest problems in the mythbusters "tests", especially the posts by Rudy and Haakon Dahl.

Chris R. said it best I think. With the rate of the a/c being constant, and the variable rate of wind resistance, it is probably better to use a/c on highway and at some point pull the windows down. If you consider stop and go traffic, there's hardly any wind resistance since you are going so slow. Mythbusters could have researched a little better.

Here is an idea, try to bust the myth with your own car. Set the trip tic to Zero, fill up. Drive as normal as possible with your windows down and when you hit empty write down your tip tic mileage. When you fill up again, reset the trip tic and drive with your windows up and a/c on. If you don't have a trip tic, write down your starting mileage and ending mileage and do the simple math to figure out the difference.

A/C consumes almost power independant of the speed of the vehicle. Also it consumes significant power compared to the total horse power of the engine. At lower speeds, the drag power component is very low when compared to the power consumed by A/C. As speed increases, the drag will eventually increase to the point where energy to overcome it will be greater than energy consumed by A/C. This should happen at very higher speeds. So the speed is the key factor here. So in real world where speeds dont exceed 100 mph, having A/C on gives less mileage as opposed to windows down.

A/C consumes almost power independant of the speed of the vehicle. Also it consumes significant power compared to the total horse power of the engine. At lower speeds, the drag power component is very low when compared to the power consumed by A/C. As speed increases, the drag will eventually increase to the point where energy to overcome it will be greater than energy consumed by A/C. This should happen at very higher speeds. So the speed is the key factor here. So in real world where speeds dont exceed 100 mph, having A/C on gives less mileage as opposed to windows down.

On the boom lift myth you should try putting a stopping point in between the supporting boxes to prevent the catapult from flipping over and falling.

The energy consumed by A/C is also relative to the outside air temperature and air moisture content. The higher the temperature, the more energy is used (and fuel). Normally, the A/C is controlled so that the cooling element does not go below 40 degrees (F), because otherwise the cooling element would freeze to a ice cube. When the cooling element reaches 40 degrees, the A/C compressor is "turned off". Only when its turned on, it consumes energy. This on/off duty cycle determines the overall energy consumed by A/C.
Hence, a hot day, it consumes the most, on the other hand, then you also enjoy it the most.

The momentary power consumed during "on"-time is again rpm-dependant, the higher the rpm, the more power. Hence, the A/C switching to "on" during overtake acceleration is noticed as a clearly reduced power. In normal situation, you hardly notice these "on/off" switches.

The speed of the vehicle is the big variable here. Rudy commented that the fuel consumption varies as the square of speed. Actually the drag force varies as the sqare of speed, and the power consumption varies as the cube of speed. As fuel consumption is based on the power delivered by the engine, the fuel consumption varies by the cube of speed also. If the test had been conducted at 50 MPH like the initial test with the fuel computer, the results would have been much different.

I own a truck without AC. I own a car with AC. If its 95 degrees outside I don't care how much power my AC consumes, its really hot and being comfortable counts for more than one or two miles per gallon. Driving in my truck at 95 mph wiht windows down is still way too hot.

There is hardly any performance loss for me when i put the A/C on. But I certainly do see the mileage drop nearly by 20 - 30%. Drag or no drag I like to save Gas so guys roll down your windows if you like to save the Gas for much cleaner and healthier environment

In response to Bruce's comment. I believe that the difference in the EPA gas mileage and the resulted gas mileage is due to the speed at which the cars were traveling. Most cars reach their optimum fuel economy when they're going at a steady 55-70 mph

I have done a highway with windows down and Ac off, and agian i done it with winows up and Ac ON...
I am really amazed to see the difference "It consumes more fuel with windows down".
The wind drag will make your car to consume extra fuel.
So turn the AC ON and enjoy a cool ride.

There are too many variables to make a proper comparison between A/C versus window down. For instance, there are a plethera of vehicles and most of those vehicles have differing types of compressor and relating systems for conditioning in a myriad of states of working, or about to break down. Each person will have to do their own computation according to their own vehicle. Another point is that the on board computers within the vehicles themselves are not intelligent enough to give the exact mpg ratio due to factors that the computer is unable to predict, i.e. ambient temperature and air pressure affecting all areas of the vehicle including coefficient of air acting on the vehicle to the tires against the road. The best way to know your mpg is to fill the tank up and write down the mileage, then drive the car until you need to fill up again, and write down the mileage again subtracting the first written reading and dividing that number by the amount of fuel it took to fill up the second time. One thing to think of also, is most cars are built to be somewhat aerodynamic and the side windows are in a bit of a eddie flow that is caused by the windshield, that eddie flow is there whether the window is up or down. The other thing to think about is that the compressor in good running condition drains 5 to 7 horses from your engine, if the engine is at 3500 rpm the percentage of fuel lost to A/C is only 0.5 percent of a 100hp engine. However, at idle, the engine is only produce 2 to 3 hp, just enogh to keep it running. Thus, when the A/C kicks on at idle, nearly 200 percent of the fuel consumed goes to the A/C. So, guys and gals, go out there and experiment, and have fun tellin' your buddies about your work.

One thing about AC affecting gas milage is that a hybrid little 4 cylinder that gets 40mpg can lost up 20mpg with the AC on where as a 5.3 liter V8 that gets 18mpg in the city might only lose 2mpg

Blake, I hate to break it to you but the majority of hybrid vehicles (a.k.a. Toyotas) all use power generated by the electric motor to drive the air conditioning (and power steering for that matter). Therefore there is no fuel loss associated to a/c in a hybrid. Beyond that, if you can find me a 5.3 L gas V8 that gets 18 mpg in the city i'll eat my hat.

I was frankly ashamed for the guys in the lack of science in their test. The first test, same car with instrumentation, showed a slight advantage to AC on/windows up. They simply didn't like the result, so instead performed a badly uncontrolled test...two different vehicles. Not only are there manufacturing variations in the engine and drive train, the worst offense is that they drained the tank, added a quantity of gas, and then ran the cars empty. I suspect there are substantial variations in the amount of residual fuel when the pickup reaches its limit.

The right test would have been the same car.

I would really like them to go back and investigate this myth some more with greater time and care.

I am actually really interesting to know the answer to this myth because I've had seemingly contradictory things happen in my own experience.

For example, when I had a tiny 1993 Ford Escort, it was obvious the A/C pulled much of the tiny's car's already minuscule hp (with A/C on, highway acceleration was a joke).

But when I had a 2005 Ford Focus, contrary to my assumptions, running the A/C seemed to have *almost no effect whatsoever* on the gas mileage. At the time, I was keeping careful time of the mileage to estimate how much I would have to budget towards gas for a summer vacation. I usually ran A/C off assuming the mileage would be noticeably better, but when I ran all A/C on (both highway and town driving), the mileage seemed almost the same.

--
Furry cows moo and decompress.

I would like to see this myth reexamined with more variables accounted for.

I have conducted test on two cars with trip computers. I did one day round trips, one way on AC; the second with the windows open. With a 2000 VW Passat, I found that on the highway the AC is best and with both the windows and the sunroof open cost an extra 15 to 25% in fuel. In a city/rural route I got virtually the same gas mileage. On short trips in a 2001 Toyota PRius the results are even stronger. In the city use windows, on the highway use AC.

As many have noted, the right question is at what speed does the break even point occur. For my Honda Civic Hybrid, it is in the vicinity of 75 mph.

Also, for those keeping track of physics: aerodynamic drag does increase as the square of speed, and power as the cube of speed. But fuel use depends on drag not on power. Think like this: fuel use, gallons/mile, is analogous to energy/mile, which adjusting for engine efficiency is, work/distance. Work/distance = (force*distance)/distance = force. The force in question is drag force.