It is a question that has been debated and tested for years: is it more fuel efficient to drive your car with the windows down than to have the windows up and use the air conditioner on a hot day? Synergetics has investigated the issue.

A simplified design of an Australian large car shape was modelled at 60 and 100km/h for front on and small cross winds.  The modelling predicted a nominal drag coefficient of 0.34.

  The results of the modelling indicated that opening a vehicle window at lower speeds was more fuel efficient than running a vehicle air conditioner.  However, at higher speeds, opening a certain combination of windows (but not all at the same time) would produce internal air flow while minimising overall vehicle drag, thus maximising fuel efficiency. 

More details of this case study can be found here.