On 30 Jun 2005 11:05:23 -0400, (Roy Smith) wrote:

Edwin Pawlowski wrote:
That's a big drop with AC. Both my cars have 3.8 liter engines and the
difference is no more than 1 mpg. Neither gets 42 mpg and struggle to get
30 on all highway

I believe most cars get better mileage on the highway with the AC
turned on and the windows closed than with the AC off and the windows
open. The added aerodynamic drag introduced by opening the windows is
worse than turning the AC on.

They did an episode of that "Mythbusters" show on that very thing. At
first it seemed like they were the same, but when they took identical
vehicles on a track and tried running them until they were out of gas,
the car running AC lost by a large margin. Seemed that the drag from
the open window wasn't nearly as much of a factor. They only tested
one make, though- I would imagine the body style has a lot to do with
how much drag is really there.

Best mileage is with the AC off and the windows up :-)