Hey, congrats...2000 posts!
Anyway, back to the theme....
When I tested mine by filling to the 'cut-off', driving over to relations in Rayleigh and back, re-filling to 'cut-off' (using same pump). I found that dividing fuel by miles travelled it equalled the computer almost exactly. 43mpg.
Mind you; that was mostly motorway travel (M1, M25) at a fairly steady 70mph, although we did have a couple of short holdups.