This might be a stupid question, but I was wondering if I close the air vents in my car or I turn on the recirculation so outside air can’t enter the cabin will it decreasd drag just like rolling up the windows? 
It’s not a stupid question, but IF it resulted in decreased drag, the reduction would so slight as to be impossible to quantify.
But, here is another question:
If it did result in slightly decreased drag, would the theoretical miniscule increase in gas mileage be worth it, considering the probability of developing heat stroke or–even worse–being poisoned by CO fumes?
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Of course it is not worth it, but I was just curious of how much it would decrease drag. I somehow imagined it to close off some vent at the front of the car therefor helping with mileage or acceleration 
Just drive along at a constant speed, watch the computer mpg calculator and switch back and forth from vent to no vent. Report back.
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I tried doing it but my car has no cruise control and the measurements are so incorrect, all over the place. That is why I’m asking because even if the gains are very miniscule I just want to know.
There was a show on something similar on Myth Busters. Only with the windows open and a/c off vs a/c on and windows closed. If I remember correctly, it did not make much of a difference.
If you want to decrease drag you are better off making sure your tires are inflated correctly or a lb. or 2 over.
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+1
That, plus buying tires with a low rolling resistance, getting rid of the junk in the trunk, maintaining the engine properly, accelerating gently, not warming-up the engine, and NOT tailgating. If the OP doesn’t know why tailgaters inevitably use more gas, he should ask us.
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Maybe a good wax and polish would make a difference. It usually makes the car run better. It is one of those mysteries never to be known.
When I was a kid, one of the car washes in town had a large sign stating that, “A clean car runs better”. 
The effect of vents open or recirc is likely un-measurable… There is likely a benefit to this but tiny.
Now adding golf-ball like dimples (3 inch diameter) all over your car WILL get you better mileage.
Mythbusters did that to a Ford Taurus and found a significant improvement. It was a huge headache to construct and looked ridiculous but it did work…
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How would that work I would think that would cause decreased air flow.
I don’t recall their explanation, but the Myth Busters guys seemed to prove the thesis that golf ball-like dimples on the car decreased drag.
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The idea is to create a small amount of turbulence to decrease drag. The turbulence reduces the detachment of air at the back of the car causing a smaller wake so less drag. Same effect as a “Gurney flap” on a race car wing except round!
Works the same on a golf ball.
The car went from 26 mpg to over 29 mpg in their test. They covered the car in clay to form the dimples and loaded the clay into the car for the “base” test.
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One way to look at it is that when the vent is closed incoming air is met by a solid wall that will impede forward motion. OTOH if the vent is open the air will hit a solid wall eventually whether the air hits the seats or the rear window. Either way the air makes a hard stop somewhere. Given the size of the vent inlet compared to the other forward facing surfaces on the car, the effect is immeasurable small.
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That reminds me of another Myth Busters test… How fast do you have to drive in the rain in a convertible with top down to keep from getting wet…
The Myth Busters did their test in 2009, but in the mid 1970’s some friends were driving on the New York State Thruway. When they started out, it was not raining, but part way through, it started to rain, but since they were going pretty fast and they were not getting wet, they left the top down and drove on in the rain… That is until it was time to exit. While exiting, they tried to put the roof up, but they were still going too fast for the roof to raise up and the wind bent the mechanism and the front part of the roof that attached to the windshield no longer met up. They ultimately had to tie the roof down with a rope. After it was “repaired” it never worked right again… And that’s no Myth… L 
L…
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I do not know about better milage, but the dimples on a golf ball has it’s own trade secrets as the dimples allow the golf ball to fly farther… Some thing I imagine that Thelma and Louise should have looked into… L L …
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Golf balls at first were designed to be smooth spheres. Experienced golfers soon learned these smooth balls would go further and straighter after they has been used a few rounds and their surfaces battered and roughened up. Golf ball companies determined the roughened surface improved the airflow behavior. Then they had to figure out a way to make new golf balls with a rough surface. Dimples.
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