Last week I kept it to 60mph on the motorway part of my 35 mile commute and got approx 56mpg over a single tankful of fuel. Bear in mind that the motorway part is only about 1/2 of the total trip.
I realised recently that one reason my MPG was getting worse was that I was accelerating faster, and changing up at higher revs than when I first got the car. I now try to change up at 2000, except when I occasionally need to put my foot down to overtake, and the MPG has gone back to its previous level.
About 4 week ago I went from Stockport to Hull which was almost all motorway. The car was unladen with just myself and my 8 yr old son. I set the trip computer before leaving home and kept to 55mph almost all the way. When I got back home I checked the mpg and was gobsmacked as it said I got 68.1 mpg...........well chuffed
Best I've ever got is 64.9, but that was fully laden. However I've not got anywhere near that since. In fact the MPG is getting worse. Used to get around 48-50mpg on my weekly commute, but now get around 45mpg.