Just thought I'd chuck this one on here as it seems to be something asked over many BMW forums so figured a demonstration may be of help.
Over the last 4-6 weeks the MPG on my E91 320D has fallen through the floor (I've not long hit 140,000 miles). I'm barely seeing 33mpg on my daily commute which was in the early 40's not that long ago & my motorway runs are lucky to push over 40 which were easily 55+ once upon a time.
Had a spare hour today so I paid attention to how to access the hidden menus & the coolant temp in the E90's (E91, E92 & E93's) & here's my result after nearly an hours drive.
The highest it reached was 73 degrees when I hit a bit of traffic but other than that it was only bouncing between 67-69 degrees, should be operating at about 90 degrees.
I think I can conclude that my stat is FUBAR & the DERV's don't like running cold!
Will replace both coolant & EGR stat in a week or two so I'll post the comparative results when I do, I'm pretty confident that this is my problem though & I hope this helps anyone having the same issue!