Hi all

I've kept a track of fuel consumption pump fill to fill, always noticeably less than display. Had a 240mile run up A1 yesterday, ill to fill 39mpg on the dot. Car reckons 45mpg.
Is it just known fact the car is very optimistic ? ? ?

Mine is optimistic, but only by 1-2mpg. There are other posts referring to optimistic trip computers too, so it appears optimism is typical. 6mpg seems a bit much though.

According to Topix (section 413):-

Average Fuel Consumption
The instrument cluster receives 2 fuel tank level readings on the medium speed CAN bus via the CJB. The instrument cluster calculates an average value of the amount of usable fuel in the tank from the 2 signals. Every 200 meters of vehicle travel the instrument cluster re-calculates this value to determine how much fuel has been used. This value is then averaged over the last 31 miles (50 kilometers) of travel and displayed in the trip computer.
The distance traveled signal originates in the ABS module and is received by the instrument cluster over the medium speed
CAN bus via the CJB.

Personally the average mpg on my computer has always been very close to the brim-to-brim calculated mpg, and I have never been able 'prove' the calculation method suggested above i.e. calculated over the last 31m. To me it always seems to work as if the calculation works on the average fuel consumption since I last reset the computer.

Pete 2007 TD4 HSE Auto

Previous FL2 (MY07 TD4): average over 18000 miles 32.86, according to car it was 34.something....not far off.
Current FL2 (MY11 SD4): average over 2000 miles 31.22, car's reading 33.4.

Slightly optimistic but not too far off - certainly not as optimistic as car manufacturers' claimed figures!  2011 SD4 HSE Auto
(Previously 2007 TD4 XS Manual)