...Looking at the fuel statistics for my W202 C230K , I can say that my consumption varies normally by + or - 8% from the median (most common) value excluding longer motorway trips. So, given that there is so much variance due to traffic, right foot, wind, road type, overtaking frequency, using kickdown and so on, I think it is impossible to test claims "on the road" - only in the laboratory. Just looking at my statistics, each following tank of fuel might be used more or less efficiently and you can never really predict this BEFORE you start.
My 5p from a total sceptic
Les