Driving slower definitely gives better fuel mileage, as long as I’m going fast enough to get it into 6th gear. I don’t measure in stop and go city driving because I do so little of it, and I rarely roll my windows down at highway speeds because I don’t like the buffeting of the wind… so unfortunately I don’t have data on either of those. All my data comes from my drive home on backroads through mostly farmland.

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.