Where do you stop? I have an Ultragauge and generally with my Mazda 5...
To drive 70km/h, I need ~5.8L/100km (give or take) to maintain speed.
80km/h, 6.2L/100km.
90km/h, 6.7L/100km
100km/h, 7.3L/100km
110km/h, 8.1L/100km
120km/h, 9.0L/100km.
I think to do 140km/h would take 10.5L/100km.
I could see 9.0L/100km being the real world gas mileage, then I can be the super hypermiler that gets 6.7L/100km.

But then what about guys who like to do 140km/h?
Even say for a short 5km trip, I can manage to nudge down gas consumption to 10L/100km, while my wife is up in the 11.5L/100km range. In three weeks I spent in Banff, I used 19.5L/100km... 11L of gas for 57km. Do I sue Mazda for grossly underestimate city mileage at 10.6L/100km?