I've found that if i drive in ECO mode and do a non-town driving journey say on A, B roads and dual carriageways, then i can get something like 52+ miles according to the OBC. Over several recent ECO driving trips between Bristol & Frome over a total mileage of 400 ish miles the 'saving' was coming up as around 10 miles, although this did involve a couple of 12 mile round trips in and out of Bristol too.
However, on my last journey back from Frome to Bristol, i thought i'd see if i could still get the same 52 mpg in comfort mode, and the answer is yes, i could. I did this by driving as though i was still in ECO mode i.e. slow acceleration, anticipating stops, and gradual slow downs. So what this demonstrated to me is that ECO mode is as much about driving style and keeping the revs down, as encouraging a change in driving behaviour. So now i'm of the opinon that i might as well leave it in comfort mode and just drive like a nun, but with the advantage of better throttle response always available when i want to see dust in the rear view mirror, as i can get more or less the same mpg in comfort mode. I still can't help feel though that in stop-start traffic, ECO is better, so i might stick with that in traffic. I don't know to what extent some of these extra miles are gained because the air-con is being turned down in ECO mode.
On a slight tangent, i've got a 328i auto loaner these last couple of days and that feels a hugely different and spirited car to drive. Tried the 328i ECO mode in traffic and discovered that the 'creep speed' in ECO is slower than in comfort mode, so maybe there is a parallel in the 320d as well. BUT the big difference is that the 328i just feels so much faster than the 320d, and i can only put this down to my frugal old man OAP ECO driving style in my 320d! I also think that the 328i auto makes a huge difference as i sometimes find in the 320d manual i'm in the wrong gear, particularly when pulling out of any kind of bend/corner. I'll definitely pick auto transmission next time.
Funnily enough the GF at the weekend said 'Why don't you put your foot down and have some fun driving as you've got a powerful car?' (we were in the 320d at the time). Well i did for about a mile before catching up with some slow moving traffic. But it's that (bloody) ECO button which has definitely impacted on my driving style, and what the 328i loaner has taught me is that beemers are about driving a mean machine and not about gaining an extra 10 miles in 400. I mean, if this saving was so important then surely i should have bought the ED variant?!! Or even a Prius!! And saved a barrow full of spondulicks. I'm not therefore convinced that saving a couple of quid in a tankful is what owning a BMW is all about, although every little helps and each to their own.
So in a strange kind of way, i wish that the ECO/comfort/sport mode button wasn't there so that i could just drive the car, like i did in my old E46. Then again, i didn't go for the 330d because i thought it would be 'too fast' for me, on the basis that 'speed kills, as well as speed thrills'.
When i get my 320d back, i think i'm going to have to RELEARN just what fun driving a BMW is and undo the ECO brain washing, and a 320d is by no means a slow car.