It seems counterintuitive, but my mileage seems better on my way home from work, when I have the windows and sunroof open. Completely open. It's Spring, and I haven't needed to use the AC yet. Anyone else notice this?

 

Hmmm...strangely enough YES you are right. I am also experiencing about .2-.3L/100kms improvement too. What year is your HAH?

 

That may be the case, however, you also might what to check to see if time of day/outside temp has anything to do with your difference in MPG. Also, you can hit the vent button without turning on A/C and still get air.

I do notice a difference in MPG on my way home at noon then on my way to work at 2AM driving the same 55 miles each way with no traffic.