I would think it is impossible to give a categorical answer for a fairly obvious reason: the answer depends on variables that differ from car to car. Whether windows down is more efficient than running the a/c will depend on the design of the car and its a/c unit, so is going to be vehicle-specific rather than a universal rule.
The big questions are: How much does rolling the windows down increase drag, and how much does running the a/c create drag on the engine.
Opening the windows on a car with a large amount of drag will make a small percent change, while opening the windows on a car with very low drag will be a larger percent change.
The amount of power needed to run the a/c will vary depending on the size and efficiency of the a/c system.
I’m not sure that having a bigger engine is, itself, a relevant variable, though. (Cecil mentioned as a point of comparison between his own tests and those in Florida.) On the one hand, a bigger engine would be providing more power, but whether that power is being used to run the a/c or overcome aerodynamic drag is irrelevant, the question is how much. I would posit the main effect of a larger engine is that it will take a larger percent change in fuel economy to be noticeable in cars where mileage was already low. If we can measure mileage to one decimal place and the car normally gets 10mpg (windows up. a/c off), you need a full percent change just to be measurable, while a car that normally gets 33mpg would need one third of one percent.
But high-fuel-efficiency cars also tend to be low-drag cars (for obvious reasons).
While the amount of the total drag determines how much power is needed to overcome it, the amount of the change in the drag will determine the change in the power needed to overcome it. The percentage change in the drag will roughly reflect the percentage change in power used to overcome drag.
Adding a fixed amount to the drag of a car will have more of an impact on fuel economy if the car had low drag to begin with than if it had high drag. At least, I think it would.
I have a ScanGauge II hooked up to my car. It’s many functions include gasoline usage in gallons per hour used plus current and average gas mileage.
On both my wife’s 2012 Nissan Altima and my 2007 Kia Spectra5, opening or closing the windows makes little difference in the gas mileage. It’s slightly worse with all 4-windows down. Of course having the AC on drops the mileage about about 3 mph.
Therefore if I’m driving in warm weather without the AC on, at least one window is down and another is crack open an inch or two to prevent interior buffeting. The interior fan is probably cranked up a lot also.
Drafting behind a semi helps with the closer the better. However that’s just being stupid. Still there’s some benefit even at safe distances where you can see the driver’s mirror meaning he can see you.
Whenever you are on the slope and the AC is on then one should always close off the Ac’s so that the car could give a better mileage and it doesn’t consume too much petrol and also the battery doesn’t have much load on itself while climbing an upwards slope.
You’re a bit behind. They revisited that myth in 2005 and clarified the results they’d found. In the original myth, they first tested at 45 mph, then tried again at 55 mph; the first test found that the windows were more efficient; the second that the AC was. They threw out the first test as they felt their methodology was flawed, and decreed that AC was more efficient. But what actually happened is that they changed too many variables. From Wikipedia: “The fundamental flaw in the MythBusters’ test was that the point where the drag becomes powerful enough to inhibit a car’s performance with windows down was inside their 45–55 mph margin at 50 mph. Going less than 50 mph it is more efficient to leave the windows down, but going greater than 50 mph it is more efficient to use the A/C.”
What they failed to mention, even in the revisit episode, is that the exact crossover point depends on several of the car’s specific characteristics, particularly its shape.
Powers &8^]
I’ve have never gotten better mileage w/AC on. It’s been the same way on over a 1/2 dozen cars and trucks. More importantly, the heat generated by running the AC will kill an engine. That’s because the hot AC Condenser coils are mounted in front of the radiator. After blowing a few head gaskets, and having one transmission burnout, I decided 20 years ago that it’s far, far more costly to run the AC. If auto designers would separate condensers from radiators, it’d be better. School buses do, but they have the extra room to do it.