^ I THOUGHT that's what i found too... but when i watched it last they came out w/ AC on top (but only with something plugged into the car). I guess the one i saw the first time was actually a revisit.

But yes, i too always get better gas mileage with windows down. But I drive the car hard, so with AC on i'm sure I'm really making a dent in the gas.

 

One thing to note with the Myth Busters test is it was done with a large V8. I believe the AC will a larger impact on a smaller engine. I have noted this with my cars. I have a 99 Prelude and a 99 z28 (saving for the xB). When I use the AC heavily with the Prelude I see a 3-4 mpg drop with the z28 I see ~1 mpg drop.

 

In stop and go traffic, or with constantly accelerating/decellerating to whip in and out of traffic, then yes, a/c off. However, with constant speed above 40 mph (below 40 aerodynamics plays a very minor roll) a/c on, windows up will yeild better milage.

The difference comes from this: what's the engine doing? And there are 3 things it will be doing, overcomming inertia (constant velocity to different velocity), punching through the wind, or combination of the 2 (accelerating at speed, say to pass someone). If the engine is spending it's time overcomming inertia, then you want as little drain on the engine as possible. However, if the engine is just punching the car through the wall of wind (constant speed) then you want as little drag as possible. With combination of the two, overcomming inertia takes the greater work, in part because of the force of the wind acting against the acceleration.


And btw, if you any doubts about this, those of you with the new B can validate this with your instant MPG reader. Notice how going from 0-30 it's showing about 5 mpg under normal acceleration? Overcomming the inertia of 3k lb takes a load of work in otherwords.



This concludes today's summer physics lesson.

 

I've never had as good gas mileage on the highway with the a/c on as opposed to the windows down.