In my experience some people will not give you accurate figures partly because they don't know how to do it properly or possibly because they have one good run, they wrongly think those figures are correct.
I could say that I get around 34mpg out of my 2.8JTD loaded at 3500kg or over

as I did from Staffordshire to Dover on all motorway and the roads were running pretty good
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I could say only 21 mpg as I did on one bad run

But that would not be the full story

On my recent 2300mile trip to France with all sorts of roads and all non paege and quite a lot of stop start in towns, we averaged 28mpg
Last year we averaged 27mpg for our summer holidays so we are pretty consistent.
My average mpg now is confirmed at 28mpg and I am very happy at the weight that we run.