I have this vague notion from my childhood that if, on a long automobile trip, you drive with the windows rolled down or with the air conditioning on, you are committing a cardinal sin. You might as well be the captain of the Exxon Valdez, for all the fuel you are wasting.
But has anyone ever quantified this? How much of a difference does it make? Is it less of a difference now than it was in the past?
IIRC, this was covered in one of the “Straight Dope” books, and was ruled “inconclusive.”
Couldn’t find it with the “new and improved search engine,” but my own incompetence could be coming into play.
Here’s the article from the Master:
Cecil’s tests were minimal–300 miles for each? That’s gotta be - what - 9 gallons of gas? How’d he measure that? By when the pump shut off? Anyway, the contrary data from Florida was at a significantly higher speed, and drag would be a larger factor.
Thanks.
Hmmm. Seems like it doesn’t make an appreciable difference which way you go.
I sometimes drive with the windows DOWN and the AC ON - I blow the AC across my brow so my head keeps cool (less road rage) but get the illusion that it’s a nice, cool breezy day by having the windows down. And my car has a nice lower vent that can blow up my skirt if it’s short enough, to keep my “thighs” cool. 
And I don’t care how much extra gas I use either (just call me “Hazelwood” and show me to the Valdez).
I notice a drop in mileage when I do this, but it’s by no means a scientific test. Your assumption about “no significant difference” is most likely correct.
There is an easy (but not-so-accurate)way to test this:
Find a long, flat road. Hold the throttle in a constant position at, say, 75 mph. With the windows rolled up, turn on the AC. Note drop in speed A.
Turn around and go back to where you came from. Drive the same stretch of road in the same direction at the same speed. Roll down as many windows as you would if it were a hot day. Note drop in speed W.
If A>W, then you should roll down your windows to stay cool. If A<W, then you should turn on the AC. There are lots of problems with repeatability here, but it could be fun if you are on a long road trip.
Anthros
There is a name for ppl that go about with the windows up and the A/C off. Corpses. At least if you do that in the summer on a hot day.
As one who used to drive many, many miles a day, I might have some results.
When I drove a small truck, the difference in gas consumption was noticeable. The a/c actually drew the motor down when on. I estimate that it probably consumed as much as 3 to 4 gallons out of a 15 gallon tank in normal city driving. I was driving roughly 100 miles a day, most of it stop and go.
When I drove a van, with a bigger engine, I would say that it used probably an extra gallon a day out of a 20 gallon tank.
Now, speed also affected the gas consumption. Go over 50 (like I used to) and one could almost watch the fuel level drop in the van. (1994 Ford Cargo Van. Piece of crap!)
One small truck I drove, a Mazda, was pretty good on gas with the a/c on. An Isuzu Pup was not all that good, but it had a 5th gear for use which saved a lot of gas.
If putting on your a/c noticeably draws your engine down, then you will use more gas. Most cars and trucks today have a ‘regulator’ so when you turn on the a/c, a piston hits the gas control and marginally increases the rpm’s. Some cars, with good engineering, have a slip clutch in the a/c, so when you have to accelerate, the a/c cuts out, giving you full power when you need it most, and when you stop accelerating, it cuts back in.