Mythbusters revisited AC vs windows down and at the speed they were driving it was cheaper "windows down" but--at 50 MPH (80 kph) the opposite was true.
I did an experiment on my most recent trip (5000 kilometers) and found no difference in mileage whether I was using my generator and roof air vs dash air. The variation s were Roof air only, dash air only, and both together. YMMV *grin*.
QUOTE(gwbischoff @ Jun 27 2007, 03:05 PM)
Don't you guys watch "Mythbusters"?
They actually did this using an SUV but the principle is the same.
Windows down and no A/C beat windows up with A/C.
They also did an episode about tailgaiting an 18-wheeler inorder to draft and save fuel and the results were amazing (and frightening).
They saw about a 60% reduction in fuel usage. O'course they had to get to within 2 feet of the rear bumper or a tractor trailer.
They have a lot of useful information on that show. Like what's the quickest way to cool a 6-pack.