My car has the fuel mileage thing in the infotainment system (2019 Volkswagen)... so I've tried looking at it on days when I drive with my windows down vs with the AC on. Oddly enough, on a ~30 minute commute at mostly 50mph, I get better fuel mileage with the windows down and AC off. It's even more odd because I feel like I drive more "spiritedly" when I have the windows down. It's not a huge difference, but 2mpg is still better fuel economy.
That said, I still drive with the windows up and AC on most days because summer humidity sucks here.