Max is right. When I was with the EPA I was a team member at the TRC track in Ohio (now owned by Honda) and we ran instrumented cars on a defined driving schedule on the track. One of the things we tested was the AC on vs. windows down thing. There was definitely a point where the AC on beat windows down every time. That point varied with the drag (air resistance) of the body in question. Most pickups found their AC "happy place" around 45 MPH, most cars about 35 or so. There are a ton of variables including tire type and so on. Before anyone asks, we only ran in very limited ambient conditions and we waited frequently for the wind speed to drop, the temp to warm up, etc. One of the biggest issues with on-road testing is attaining repeatable conditions.

I can easily bore you with WAY more than you ever wanted to know about this stuff!

Dan