When asking for submissions of MPG figures you state: "Work out your average fuel economy over the last month. You can do this from your trip computer consumption meter average, or you can do it on a ‘brim to brim’ basis."

I've been doing both on my recently acquired 2006 C5 1.6HDi estate using both methods, zeroing the computer on the forecourt each time, but they disagree!

Computer says: 51.3, fillup to fillup says 49.6
50.3 is 48.13
51.3 is 48.7
53.2 is 49.01
53.2 is 50.9

If I eradicate any minor fill up differences by combining those last 5 fillups as if they were one the average figure is 49.45 which is between approx 1 and 4 mpg worse than the car is lying to me. Total distance is 1,960 miles. Total fuel, 180.16 litres.

My question therefore is how can I have any confidence in the Honest John mpg figures if they have been supplied to an unknown extent by inaccurate computer readouts, which like the official mpg figures in my case at least seem also prone to convenient exaggeration? Even accounting for the fact that the computer despite using decimal places, in fact changes in steps of about 0.9 or thereabouts, the range is still a significant error of between approximately 2% to 8% optimistic for the computer.

Thank you.