Here in our country where it's usually around 35C, I keep it a point to put the windows down when i'm the only one in the car. Since i have window visors on, I open my window only slightly lower than the bottom of the visor so i can have some air and at the same time, more engine power.

 

I watched an episode of Mythbusters once and they did a test on this. Having your windows down produces drag. However, they did most of their testing on highway speeds, so if you're driving on the main roads, I would probably just roll the windows down.

 

Around the city I cruise windows down. On the freeway it's usually cracked so that the windows are still under my visors, or A/C, depending on whether or not I'm going uphill.

 

Mythbusters' mistakes in AC vs windows down episode — Autoblog Green

 

FYI, no one should be listening to this guy. If you adjust music higher when windows are open, *that* may cause damage.

High frequency noises can damage your hearing. Not low frequency.

 

Wind noise is full spectrum. After 48 years of riding mostly quiet motorcycles, I have significant hearing loss and tinnitus 24x7, mostly from wind noise. Unfortunately, I didn't discover foam ear plugs until 1995.

 

This is incorrect. All frequencies can cause hearing loss.

 

The V.A. audiologist that fitted me with hearing aids categorizes high frequency loss as classic combat hearing loss and low frequency as occupational hearing loss.... I have both in my left ear, a lesser amount of low frequency loss in my right ear and tinnitus.... After testing me he said he could tell I had been exposed to a great deal of combat and had spent a lot of time driving with my window down.

 

Since this question is regarding MPG, the answer is open windows. From my experience the A/C on the lowest setting will cause the engine to lose a great deal of power - as soon as it's on the car will feel like there's 2-3 people sitting in the car going uphill. Since the little engine gets significantly bogged down, it'll take more throttle to drive around, meaning more gas used. Definitely only use it if you have to.

 

I've heard a million times how driving with your window or windows open creates so much drag, you're better off using your A/C. I try to keep windows up, and cool the car off with the A/C then turn off the A/C, recirculate the cold air and turn the A/C back on when needed again. Although over 90% of the A/C season I'm getting from here to there on a motorbike, bicycle, public transportation or my feet.

 

Once you are up to 70 the windows being down will create some major drag.... If you have dark tint and The Mugen type vent visors they will help keep you cool with the windows slightly lowered and the A/C off when the temperature isn't much over 80 degrees and not create as much drag.