I just drove home from work the other day with the AC on for the first time in my 2009 TDI. I was happy to see the mpg was 48 after my 35 mile ride. I drive 55 mph which definitely helps. Without the AC on, my best mpg has been 50.0 for a 35 mile trip. My car has 4900+ miles so it is still being broken in.
The old rule of thumb back 20 or so years ago was that Air Conditioning took about 10 horsepower to run which would work out to roughly 1 gallon per hour for a gasoline engine and about half a gallon per hour for a diesel. Technology has improved since then, but the newer refrigerants aren't nearly as effective as freon so I'm guessing that the conversion there may be negligible.
It appears that idrive did considerably better than that in his application since a 2.0 mpg loss would work out to a 4 percent cost which would work out to about 0.044 gallons per hour. That figure seems too low to me from a mechanical and practical standpoint as it works out to a little less than 10 percent of the historical estimate but maybe VW has engineered it such that an amazing feat has turned into reality.
I just ran rough numbers on my current vehicle and I think I'm using about 0.3 gallons per hour to run my A/C on gasoline so maybe there has been a breakthrough for the climate control industry.
This is what I found on TDI club about the new style AC systems. Fan setting 1, A/C on results in better fuel economy...WHY? simple the ECU commands more timing advance than it normally allowed negating the effects of the slightly higher load on the engine. Also running at fan setting 1, reduces the evaporative rate of the freon thus LOWERING the total load on the motor maximizing the benefit of the more advanced injection timing.
Also the slightly higher load causes a resulting decrease in required EGR rates again slightly boosting efficiency (albeit very slight benefit).
A/C Compressors (VDP= Constant pressure, variable displacement) DO NOT OPERATE WITH A CONSTANT LOAD despite always being engaged. Load is highest when first turned on dropping off as system pressure stabilizes based on evaporative rates (determined by fan settings in the cabin).
More detail: Once the system pressure stabilizes and with a constant fan speed setting the load will drop off over approx 15 minutes and the interior is cooled. Using Recirc reduces load further by limiting the evaporation rates due to having to cool the colder air of the cars interior. However this isn't the healthiest setting given the gradual reduction on oxygen and increased CO2 from the occupants breathing.
If you are too cheap to use A/C drive a bike because you can't afford to drive a car. A/C after the system stabilizes reduces fuel economy so little that it would be lost in the margin of error for the purpose of measuring fuel economy.
Higher fan settings will result in measurable increases in fuel consumption, again the key is the lowest setting and allowing stabilized operation for as long as possible.
Study up on "VDP" compressors, ECU mapping for the engine when the A/C is selected "ON" as well as the effects of load and EGR duty cycles.