Gas Mileage: A/C vs. No A/C

Does any SD have info on whether or not A/C burns more gas than rolling down the windows? At high speeds, for example, I hate the excess noise from open windows so I prefer the A/C - such as on a day of borderline temps. Many argue it’s wasteful, but rolled-down windows creates more drag, so which is worse? (Let’s assume we hold all other variables fixed, such as speed and chosen highway.) Any studies on this, perhaps? - Jinx

Any recent studies?

Cecil addresses it here

Looks like a wash.

Different cars and different driving conditions would change the stats.

Wind drag increases with speed wile A/C puts an almost constant drag on the engine. At lower speeds open windows give you better mileage (and less noise), as speed increases wind drag overcomes drag from the A/C. Although the speed at witch the transfer varies from car to car, it’s usualy around 45 MPH. In any case when wind noise becomes to much it’s time to roll 'em up and crank up the air.

This is anecdotal, but when I was dating a girl in Orange County (about 140 miles one way), I noticed a fairly marked improvement in gas mileage after I fixed the A/C in my car.

This was in a '93 BMW 325, averaging a little over 70MPH.

There have been much better studies than Cecil’s.

New Scientist looked at this earlier this year.

They linked to a .pdf of a study from the US Society of Automobile Engineers.

The link by Exapno Mapcase has it right. High drag vehicles don’t suffer much more from an open window, whereas streamlined vehicles suffer proportionally more.

There is an offseting factor in that high drag vehicles (think SUV) often have largish engines. At moderate power levels, such engines suffer high pumping losses. Adding an additional 3 hp or so load (the AC) mostly just reduces that loss, only slightly impacting fuel economy.

Coupled with the fact that an SUV may be using 50HP to maintain speed, while a slippery economy car can do so with 10, means that 3HP extra load is much more significant to the econobox.
I would LOVE to see ammonia/water sorption cycle AC applied to hybrid. Might be tricky to keep the water from freezing in winter, but it seems the effiency gain might be worth it.

Slim chance of that though. Most quto makers can’t even be bothered to valve off the heater core when the AC is activated.