If I need to be more specific:
The car is an average suv traveling at highway speed on a sunny 80°F 26.7°C day where it would get too hot inside without any cooling due to the green house effect
Comparing:
Window open: Only the driver side window cracked half way
A/C on: A/C set to a cooler 75°F 23.9°C to compensate for the weaker air flow than cracking the window
Comments
This is going to vary drastically with different cars and ambient temperature.
EDIT: I want to add a lot of people are pointing to the mythbusters which is probably the best info you are going to find on this, they found having the windows down was usually worse and thats probably as specific as you are going to find without doing an actual experiment.
Depends on the car of course but modern cars have put a lot of effort into aerodynamics and also the efficiency of HVAC. Especially if your HVAC is electric instead of being mechanically driven by the engine, the HVAC is probably more efficient
Mythbusters tested this (twice) and showed that driving with the AC is more efficient sometimes. Under 50 windows open is better, over 50 AC is better. Running the AC does introduce load on the engine but not as much as opening up windows messes up aerodynamics of the vehicle. I’m sure with older vehicles that were not all that aerodynamic and less efficient AC units and engines overall in vehicles it may have been the other way around.
Source https://mythresults.com/episode22 and https://mythresults.com/episode38
Edit: this is what I get for writing this comment in a drive through (with the windows down). As others have pointed out the efficiency changes based on the speed of the vehicle. So at above 50 MPH this is true but under 50MPH windows open is better. I fixed it above as well!
The US TV show “Mythbusters” studied AC vs window in two different episodes and found that below 50 MPH, it is more efficient to open the windows. Over 50 MPH, drag takes over and it’s more efficient to run the AC.
https://mythresults.com/episode22
https://mythresults.com/episode38
Didn’t Mythbusters have this in an episode?
Mythbusters tested this. Drag is much worse, especially in modern cars. Modern vehicle AC compressors don’t leech off as much power as older compressors
It depends on the car’s aerodynamics, the efficiency of the car’s AC system, and the speed you are traveling. There is no one answer. The one thing I can say however, is that if you just leave your AC on auto year round you very much limit the odds of getting that musty smell from the HVAC system.
AC on regardless of mpg impact. I barely notice a difference anyway