A couple of points. On the spider tuning thread there's a comment from TiRich
"Bosch MAF's in my opinion should be a service item. The effect they have on performance and MPG is very significant. A lot of people with cars over 40k miles don't realise that the MAF has gone out of spec as it, in general, happens so slowly so goes unnoticed. I discovered that cleaning the MAF as often as every 3k miles made a significant difference., it shows how sensitive they are - especially in the winter when there is a lot of road spray. A lot of the contamination in the spray ends up on the MAF."
http://www.e90post.com/forums/showth...106924&page=10
I did a fairly big run yesterday and got 41 mpg which isn't so bad but i noticed a few things.
6am in the morning, doing 70 (yes really) in 6th was giving around 30mpg on a flat surface. Three hours later the same situation was getting me 50mpg. So i assume that engine and outside temp has a big effect. With it being generally so cold at the moment, perhaps this is a bigger factor then we think.
I found it slightly better to floor it up to speed than slowly reach speed.
Doing the same in my 120d would give me over 50mpg easily but my 120 would have been on 10k and the 320 is on 60k.
So the comment about the MAF above interests me because it would make sense in my (and perhaps your) situation.
cheers
paul