On a '96 Ford Contour 4 cyl, about 2 mpg loss on highway. I do not know on my '81 MB 240D. I do think the older York and GM compressors have a bigger loss. The OP did not state which MB that had a 2 mpg loss. I do not know or can comment on AC vs windows down. I would think AC beats windows down (2-55 AC). I never did a MPG in my Bricklin with a York compressor. The HP drop was noticeable, as I remember. I did notice an HP drop on the Contour, but not as much.