[N1rve]

Originally Posted by Jyn

I should clarify, rereading the OP, my post is vague.

Using the AC compared to the vent - yes, vent will preserve MPG.

Using the AC compared to rolling down the window - negligible due to aerodynamics. Better to just use AC.

So in summary, you're correct that using AC affects MPG by a small amount (since the engine has to turn the pulley that turns the belt), but when compared to the other solution of rolling down the windows, there's not much of a difference. Our vents suck due to the layout of the parts in the engine, so it's not often we get to enjoy the vents for cooling purposes