fuel economy readings??????

[newcastle brown]

I am sure I will get some abuse for this, so here goes

on a resent full tank fill up,
on the dash reading 5.0 I am using on average 6.2l of diesel per 100 km.
on dash reading 5.1 it will give me a range of 858km.

on the normal odometer reading I am showing I have done 78.9 miles

does this mean i'm getting upward of 45+ to the gallon

does this make sense
I am sure I will be put in my place by more knowledgeable people