I know, but it is sooooo painful.

How do you account for head or tailwinds, air density differences, summer/winter fuel additives, average speed or terrain. Seems like the only way to be close to accurate is for that particular X5 that the formula is based.
What would you say the +- range is for accuracy?

Air density is covered in that link, as well as speeds vs. HP in the result set, and summer/winter fuel energy densities (approx.)

Wind can be averaged out if you drive round trip with approx. the same wind speed/direction. That's why (one of the reasons) they make you drive both directions at Bonneville Salt Flats when doing Speed Week.
Terrain is tricky. My gas mileage in Washington is going to suck compared to someone in Iowa...I suppose you could add in another factor for change in potential energy due to elevation change. Also need to account for combustion efficiency on a NA motor at higher elevations. And the lead foot effect of going uphills.

+- 50% :)

Correct, there are many considerations in the equations to achieve accuracy. It is my understanding that the intention of the calculation is something that can be universally applied for a reasonable difference in mileage one can expect from summer to winter when nothing is wrong with their X5. I stand corrected, +- 50% works for me.

Yes, all X5's being identically maintained, same tires, same gas (meaning station not brand), same highway, will have different MPG because of the foot attached to the accelerator pedal. That's where the +- 50% comes in ;)

On the m62 one, I get 450 Kms for a tank in the winter and 550 kms in the summer but that's most likely due to the traffic in summer not being as slow.
I run with no clutch fan.
I kept an eye on the oil temperature and at -20C outside temp, it barely makes it to the operation temp by the time I get to the office which is about 40 min drive, unless it's heavy traffic.

On my 4.4i ive been doin pretty decent. About 15.5-16 even with a bad ccv and the below freezing temps. LOL

Sharing me experiment but noted it was far from perfectly scientific.
Made a round trip today consistent of 175 miles each way (350 round trip).
Air temps ranged from 9-15F

On the way first leg and half of the return leg gas in tank was from Costco 91 octane. On the return leg I filled up from empty (21.87gallons) of NonOx 91 octane gas.

Mileage numbers using on board computer. Yes I'm sure it has an error factor but just consider this comparison.

At start of return leg I reset consumption average #2 a few times 10-15 miles apart while on the Costco standard winter 91 octane gas. Each time the number floated back to match the consumption 1 field of 15.6mpg.

After filling up with the NonOx 91 Octane I again reset consumption 2 a few times and then left it for the remaining 85 miles home.

I am sure there were lots of other variable factors but I would say foot position on gas pedal and speed were fairly consistent.

NonOx gas was later in the day so air temp was slightly lower (9F vs 12F). No noticeable wind, no real changes in elevation.


Results
(No photo sorry)
Consumption 2 had 70 or so miles on standard winter gas 91 Octane from regular supplier.
= 15.6 mpg

(Photo)
Consumption 2 in picture has 70 miles or so highway on NonOx 91 octane.
= 17.8 mpg


Consumption 1 in photo I reset few days ago. Has 80 mix town highway, + full tank on highway all regular 91 octane, and the 70 or so miles with NonOx. Not super relevant but I'm giving details since it's here.
http://img.tapatalk.com/d/14/02/24/4usy3uba.jpghttp://img.tapatalk.com/d/14/02/24/qabaryve.jpg

Junkcosmos-very interesting!