The guys on Myth Busters did an episode on this issue. The result was
that there is much less drag with the windows open than energy used to
run the AC with windows closed. IIRC, most compressors rob about 5
horse power from the engine. Add to that several big fan motors and
increased radiator fan on-time which puts more strain on the engine via
the alternator. It's not even close.

Keith O.
90 Westy Syncro "VikingWagen"
Past VeeDubs in the family
1984 Westy "Brownie"
1980 PU diesel
1978 Rabbit diesel
1979 Rabbit gasser
1963? 23 window Splitty
1965 Bus
1969 Bus
1959 Bug

On Aug 28, 2007, at 4:19 PM, Automatic digest processor wrote:

> Date:    Tue, 28 Aug 2007 14:10:40 -0600
> From:    Maggie Dew <wildebus@GMAIL.COM>
> Subject: Does AC Affect MPG?
>
> OK, first off let me say I want hard data - not anecdotal speculation.
>  A
> certain member of this community who shall remain unnamed (yeah, I'm
> talkin'
> about Roadhaus), has challenged my assertion that my fuel consumption
> is no
> worse (maybe even better) with the AC on and the windows up than it is
> with
> the AC off and the windows down, which adds drag.  There are, of
> course,
> several variables - highway driving is my main concern here.  Speed is
> a
> variable since I get much better mileage at 55 than I do at 75 (love
> that
> Subie).  While I keep meticulous mileage records, I have not recorded
> such
> variables as whether the AC was off or on, whether the windows were
> open or
> closed, or what my average speed was.  Guess I'll have to revise my
> record
> keeping.
>
> What say you?
>
> Maggie
> '85 Subaru-Westy "Anne Bonney"