I think that the Faria and Navman 2100 are the same thing. They look identical from the outside apart from the colour. Same layout, same functions, same buttons, same display same graphics, same sensor. I would guess that one is a badged version of the other.
My 2100 is fine, it took a bit of setting up, but is now within 0.25 litre each time I fill up, which is usually between 50 amd 75l.
"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools" Douglas Adams