Ghia Trip accuracy MPG - is yours accurate?

Has anyone done a long term accuracy test on the average MPG reading?
I am finding that mine reads say 56 MPG (UK stuff here) over 150 mile run. I fill the car to the brim each fill up and record the litres, the odometer
and the trip. I rest the trip and do it all over again. This is not just one test, but I keep a record of EVERY litre I put in it and I average out the figures over a very long period of time.
Currently, the trip reads say 56 mpg, and I actually get 48-49 MPG. This 10% exaggeration is consistent (+/- a couple of percent).
So how do I adjust the reading down by 10%?
I did a distance check on the trip and odometer too, using the KM makers on our motorways, its accuracy is better than 1%.
For our USA friends, there are 4.54 litres in a UK gallon and a KM is 0.62137 of a UK mile.
I'm not too impressed with the MPG figure as my wife has had a 1994 Peugeot 405 1900 TD for 3 years, drove 40,000 miles and averaged 48 MPG. So I was expecting 55 MPG at least. I hope its ducks arse syndrome and things will get better with time.
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My findings (2.0 Ghia Petrol 2001 UK) are that the MPG achieved on the trip computer bears little resemblance to what it costs at the pumps. The trip computer always exaggerates the fuel efficiency, but oddly enough is bloody accurate on average speed (using motorway markers) ...
When my speedo is showing constant ~66 MPH my (reset) trip shows average speed of 60 MPH, which I guess is normal, or at least within tolerance - perhaps to stop endless claims due to people getting prosecuted for speeding due to the speedo being .5% over.
Regards MPG, it will be a cold day in hell when any (UK) Ford meet's it's stated MPG figure. None of mine ever have.
Were I cynical, I'd suggest the average MPG figure shown uses the average MPH from the speedo (overstated) but the distance calculated by the computer (accurate).
-----
I wish I could afford a Merc. Sigh. C220 CDi SE Saloon, in silver, mmmmmmmm (sorry)

odometer
just
10%
on
Peugeot
was
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trip
bloody
computer
That's probably right, as my odometer seems quite accurate (although I would imagine it is difficult to gauge accuraracy to 1% using road markers). The fuel efficienty figure is always 5-10% optimistic, and I've never pushed it too far on the reading for how far I can still drive, usually losing my nerve with 70km to go.
I do match the combined rating with my TDCi, but that is with a 75% motorway (140kph) and 25% urban mix.
Stephen
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Hi, I have tested the accuracy of a Honda Jazz over many fill ups.
Results from the MPG readout are on average about 7% optimistic.
I have with some difficulty contacted Honda, and while not getting an explanation I am told that the petrol is measured by measuring how long the injectors are open. They said this was very accurate. Maybe it is. I think the problem is simply one of calibration.
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