Not used the instantaneous readout on the 1.4Ecotsi but I would certainly expect a bit more from mine at that speed on a motorway run - say 45/47 at 75/80. Could be influenced by what tyre pressures you are using or whether the AC is on. I have climate on mine, but whether that is more or less efficient than the basic system I don't know.
I checked yday, front tyres were 38.5 and rears 35 . A bit high no? These were pressures that came with the car. I lowered them to 37 and 34. Fuel flap did say 38/35, and for 'comfort' a lower figure, I think it was 35/32, but can't remember. Will see if any difference. I wonder what speed the delivery guy did on the motorway when he delivered car, he did 57mpg since start how? 55-60mph all the way?