They said a/c on, in their particular test, in their particular vehicle, on one particular day, was better. I somehow doubt that one test covers all vehicles and all duty cycles
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
Best economy is to have windows up and air-con off, if you're girlfriend complains about the excessively hot cabin try wetting her T-shirt with water. (be careful with this method, as often the following activities can result in an unwanted pregnancy which totally kills economy).
given that the power to overcome drag is a function on velocity cubed defining the speed is a slightly important. it would be hard to imagine that opening the windows on a modern German car travelling on the autobahn at 120mph+ would use less fuel than a modern efficient A/C system. it is also hard to imagine that in bumper to bumper traffic of a few MPH that opening the windows would have any meaningful effect on fuel efficiency but the parasitic loss would be measurebable