Recently watched the episode testing open window vs AC mileage. Please note that in busting this by your average Joe test you ignored a large important fact. The computer test was at 55 mph and your average Joe test at 45 mph. As parasite drag increases as the square of the speed increase, this would have a major effect on the results. Your results would be valid around town (low speed), but not on the highway. Also, at highway speeds, the engine is in a more efficient RPM range so the power required for the AC would be a much smaller percentage of total power as opposed to the power increase required to overcome the drag.
Thank you Chuck, I too just saw this episode and came up with the same conclusion. The faster you go, the more parasitic drag you'll have with the windows opened.
I believe they should have ran the computer model test at 45mph. This would have shown a lower fuel consumption with the windows down compared to traveling 55mph with the windows down.
I've been told that 50-55mph is the speed where windows vs ac come out about even. I would say this episode confirms this information.
A better test would be actually two tests. A test at 35mph and a test at 70mph. Of course they would have to do it in a non suv or on a different track. But I believe this would have shown a bigger difference between the results and I believe this would also result in an answer that there isn't only one answer to the window vs ac test.
This message has been edited. Last edited by: laughinggas,
I logged on to refute the findings of this episode, but found out that chuckt1 and lauginggas beat me to it. I was suprised that the normal attention to detail seemed to be lacking on this one. Normally all angles, regardless of how unlikely, are looked at. I am "assumeing" that time ran short durring the filming. I still think the show is great keep busting.
I believe the test is incomplete, thus providing flawed data. The tests should be done with multiple vehicles with each vehicle being used in both tests to find an average for that vehicle. Each driver should perform the test with each vehicle. Even though both vehicles are identical, the fuel consumption can be different. The difference between each vehicle may not be much but when the human element is added, the results can have a greater variance.
Use the same car for both tests, using different speeds and different AC levels... there is probably a point at which the drag from the windows and level of AC with windows up would produce a similar fuel consumption level.
in addition, i think that there is rarely a time when average joe runs the a/c at full tilt the entire time in the vehicle, and i've never, myself, kept all two/four windows down while going 70mph for longer than the initial cool down of the car cab - and i agree in using the same car for all tests, running a computer test for all speeds done, and using real-life roadways (city streets and highways, not just a track) would provide more valid results. also take into consideration the stop/start in residential 25 mph limits, and the constant 70 most go on the highway. i think three speeds should be tested, and all using the computer data as well. average joe isn't going to even be on a racetrack with a minivan, you know? so, i guess what i'm saying is a few more realistic average joe tests need to be done - definitely a revisit
If I'm not mistaken, the ac level doesn't have any difference with fuel consumption. I believe the compressor is either on or off. The compressor doesn't have varying levels.
[quote]If I'm not mistaken, the ac level doesn't have any difference with fuel consumption.[/quote]
This may vary between some vehicles, but I think the most common arrangement is for the A/C to have a clutch that runs off of the engine's fan belt. The clutch engages when cooling is demanded. The demand is dependent on the comparison between the cab temperature and the comfort setting for the A/C (and the A/C being on, of course). Setting the A/C for as cold as possible will definitely skew the test results (unless you want to be _very_ litteral about the myth). There are other variables in this test. The outside temperature, the color of the vehicle (that is, how well does the vehicle reflect/absorb the sun's radiation), and the strength of the sun.
I just thought of another variable, one that is a bit zany. Was the rear window of the SUV's openable, and was it open on the SUV with the windows open. It is obvious to me that the air resistance would be much less, and that this would not be interesting unless some vehicles have rear windows that can be opened. Come to think of it, the rear window of my pickup CAN be opened.
The one other concern I have about the test was if the gas tanks were completely empty when the 5 gallons were poured in. I have never seen a gas tank that can be pumped out completely with a hand pump like was used on the show. The only way to get a true comparision is to run the vehicles until they are out of gas and then put in the 5 gallons.
Bingo, Papaw! They didn't even check the vehicles level, they just pumped out whatever the hose reached. Had they run the vehicles out of gas on the same course they were about to use, that would have eliminated ONE of the MANY variables they disregarded. I often wonder if they are really that clueless, or if they are wrong on purpose to make it controversial.
It was this very episode that prompted me to join so I could add my $.02. There were WAY too many variables disregarded in this 'test' to make any definite conclusion. Most all have been touched on here with speed vs. resistance being the more crucial. But was a constant speed even obtained? I may have missed it, but was mention of using cruise control ever mentioned? The exact amount of fuel also brings in more questions than answers. I would not, after viewing this show, conclude anything other than more controlled testing should be done.
I have to agree. They should follow-up on that myth, and attempt the testing again, with all variables accounted for, and run at 30 and 70 mph. Keep the cars white or black. Maybe just use the same car one after the other to reduce any airflow imperfections causing drag. And I think they need to get rid of the track, that is dependent of the drivers' styles of taking turns.
I think there have been a lot of good points here, but i really like the idea of running at two different speeds, running the tank fully empty, and using the same car.
Heh, 'twould appear I'm not the only one who became a member to post something about this episode. I'd like to point out a few things I've noticed, and second a few things others have said.
1. First off, in every car I've driven, it seems like the A/C saps more torque at low speeds; that is, I notice more of a difference between flooring it with the A/C off and flooring it with it on when cruising around town (and accelerating from a stop) than I do while cruising at highway speeds.
2. I've driven a variety of different cars, everything from an anemic 1996 4-cylinder Neon to a 2007 Grand Prix rental - and it's always seemed as though the A/C takes LESS power (relative to the engine's total power) from a larger, more powerful engine than from a smaller one. I only noticed the difference the A/C made in engine power around town in my V6 Malibu (which I drove for two years), whereas in that old Neon I had to turn off the A/C if I wanted to merge onto the highway or pass a car. The point is this: a vehicle with a smaller engine, like a compact car or a midsize with a small 4-cylinder, should notice a greater difference in fuel economy with the A/C off than would a larger vehicle with a more powerful engine (like a V6 or V8).
3. Different cars have different aerodynamic properties, and the airflow is disturbed differently by rolling down the windows from vehicle to vehicle. A vehicle with all the aerodynamic finesse of a brick (like an SUV) might not lose much (since it has little to begin with) by rolling down the windows, while a more streamlined vehicle might notice a more marked decrease in economy.
4. As has already been pointed out, air resistance isn't nearly as much a factor at around-town speeds as it is on the interstate.
5. Cruise-control would be a must for this test, I'd think, to ensure a constant throttle setting and speed.
6. A computer model test should definitely have been done at 45mph for control purposes.
All told, I was really very surprised at the lack of controls and testing of possibilities for this myth . . . seems as though others get tested beyond the realm of reasonable possibility, while this one was done almost haphazardly.
Now, that said, my 7.5 year-old nephew and I both love the show, and we have the DVR set to automatically record every single episode. Everyone makes mistakes sometimes, but you've still got a good show. ^^
You guys all know that they said on an episode that they changed their results and that there is a speed at which it is more efficient to drive with windows up and AC on. They said it is 55mph I think.
It really depends on the type of vehicle and the age of the vehicle. If the A/C compressor is around a decade old, it will place way more drag on the accessory belt of the motor, taking away a lot of horsepower and decreasing gas mileage signifigantly. I know this from experience with my 92 GMC pickup. When using the A/C, I lost tons of power and got about 12 mpg. With the windows down, I got about 15 mpg, by the way, I always use the same gas station, same nozzle and always stop at auto-shutoff. I like to keep a close eye on my mpg as it can tell me when something is wrong with the truck. Anyhow, on my newer 98 Bravada, using the A/C uses about the same amount of fuel as having the windows down. This is driving in a 2 mile 30 MPH area, 1 mile 45 MPH area, 3 mile 55 MPH area and then 2 more miles at 45 MPH. A/C-on mpg vs. Open Windows-mpg, varies from car to car and there really can't be any definite answer on this for every vehicle, just the one they choose to drive.
