i found this:
"Over the years the speed of 56mph has often been talked about as being the optimum speed. This was due to the old fuel consumption test being run at three speeds: urban, 56mph and 75mph – and 56mph was always, unsurprisingly, the most efficient of these. Typically, cars are most efficient at 45-50mph."
How to save fuel - the ultimate guide | RAC Drive
8 industry-expert tips for how to save fuel: looking at cruise control, the speed you drive, what gear you drive in and how much fuel you carrywww.rac.co.uk
my car has an average and an instantaneous readout on fuel efficiency, and the average is the one i usually leave showing on the display. i think it's averaged 5.5l/100km over the last couple of thousand kilometres, but most driving we do is open road.
There's one part of that article that is a bit misleading
AC does increase fuel consumption
HOWEVER
Using AC instead of opening windows if you've to drive a distance is gonna be more efficient.
Also on a side note
You know those cars with 5 star ratings for safety?
No car on earth with a safety rating is ever tested above 65kmh to get that rating.