A couple notes on AC:
1) If you have the standard type of AC (not automatic climate control where you just set a temp) it pretty much uses the same amount of power no matter what the temp setting. It just mixes in warmer air when not on full cold (at least this was how my last car worked). Therefore, the most efficient way to use it is put the temperature setting all the way to the coldest and turn it off and on to control the temp as opposed to running it continuously and using the temp knob to adjust the temperature. This is probably obvious to most but JIC.
2) For MOST cars, the point at which air resistance passes rolling resistance to become the primary cause of drag is right about 55 mph. By 70, it becomes quite a force and it grows exponentially. At that point, rolling down your windows or doing anything else that disrupts the aero of your car has quite an impact. Sounds like some drivers are not seeing that so maybe the Elantra is different. But it also means that UNDER 55 you probably are just fine with the windows down and the AC off as drag isn't really much of an factor at those speeds compared to rolling resistance.
3) Its easy to forget but if you have a sunroof, closing the internal cover to block sunlight from above during the day can have a profound effect on the cabin temperature with 0 cost in mpg.
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