A/C and fuel efficency?

The Mythbusters showed us that it is more fuel efficient to run the air conditioner in your car than to drive with the windows down.  I usually open all the windows for a block or so to let the hot midwestern summer swamp-ish air out of the car, and then roll 'em up and put the A/C on low.  My internal debate is this:  Is it better to run the A/C with or without recirculation?  It seems logical that cooling air that is already cool would require less energy, but I also wonder if the fan has to work harder to recirculate the air versus just letting the air rushing by outside the car in.  Does anyone know or have any connections at M5?

That's not necessarily true - it depends on what speed you're going.  If you're going at a relatively slow speed, there is minimal drag created by lowering your windows, and you'll have better fuel efficiency than with the A/C on.

Once you get up around highway speeds, the drag from lowering your windows increases, and you're better off having the A/C on.

It's better to run the A/C with the recirculation on.  It takes less energy to recirculate already cooled air than it does to cool more outside air.

 Thanks Dana!  I've always wondered...So city driving  = windows, and freeway driving = a/c.  nice rule of thumb

Yeah, generally speaking that's a good rule.  It's a little tricky because every car has a break-even point speed where A/C becomes more efficient than open windows, but it's different for every car.  I believe generally the less aerodynamic a car is, the less difference the drag from air coming through the open window will make.  So for example a boxy SUV will be better off with the windows open at a higher speed than a Prius.

But since nobody has quantified this break-even speed for the various different car models, you're pretty safe to use the highway speed rule of thumb.