I haven't driven my 240D much. So I don't know what type of milage it should get. Just a few question, what type of driving is this, city, highway, mixed, country roads?
I'm not an expert on A/C stuff. Still, from what I gather the A/C compressor on these cars consume a lot of power. I think about 10-15 hp. On my Ford Contour I think I loose about 2 mpg on the highway with the A/C on. I think the newer A/C compressors use about 3-5 hp. Also, using 134a has higher head pressure on the system, so more power needed to run it. At least that's what I'm told.
So if you running A/C a lot 15/5= 3 3 X 2 mpg = 6 mpg loss. If the 134a uses more power to run it, then it would be more than that. From what I read the 240D is about 26 city and 30-32 highway. So if your driving city or a mixture then your probably down to low 20's.
Tom