BMW Fuel Computer - Inaccurate, any ideas?

[Sortie 10]

Fuel consumption not a normal topic for Pistonheads!
I've been running a check on the consumption of my Z4 2.5 SMG over the last 6000 miles. The fuel consumption shown on the computer (I reset at every fill) is consistently 2 mpg greater than my spreadsheet shows. I have double-checked my formula, so there could be a problem in the electrics, any body got any ideas?(Incidentally my consumption has averaged just under 26mpg over that distance including two Alpine hoons, best of 32.4, worst of 21.7)

[WingChunDave]

Perhaps the computer only reads the consumption when you're on the move and not the fuel the car uses whilst the engine is running or sitting in traffic? Could this account for the difference?

I have to say I don't pay too much attention to mine, only that my 325 averaged 27 and so far my 328 averages 26 and dropping!

[robdickinson]

Wouldnt supprise me if the 'innacuracy' was one on purpose to make the owner feel happier.

[slim_boy_fat]

I thought there was a way to calibrate the fuel computer, ie. multiply by some factor to correct the display to what you actually are getting.

[StuB]

Ignore the computer

[jj.]

my 5 series OBC is normally about 1-2MPG out on a full tank c.415 miles for the 3ltr.

However the speed readout on the OBC (when on cruise) is about 3MPH below the speedo, and another 4-5MPH above true speed. (i.e True-GPS speed of 70MPH, OBC says 75MPH, speedo says 77MPH.)
jj