Driving efficiency with windows or sunroof open?

[chibi_kurochan]

I believe you're referring to buffeting. This happens with every aerodynamic car and sometimes non-aerodynamic cars. It's caused by the laminar airflow being disturbed, and happens when you open the windows more than some amount (likely varies somewhat by individual car).

In the Model S, some folks have also had buffeting when the hatch stoppers are not correctly adjusted.

Hmm I see.. and what is this hatch stopper you're referring to?

 

[jerry33]

Hmm I see.. and what is this hatch stopper you're referring to?

See post #17 here

 

[supratachophobia]

This was a Mythbusters already. At low speeds, windows open, at high speeds AC. At least for ICE.