Oh, and other odd thing: I had the chance to drive 450 km on a BMW 316i (E46), and I've done it with 30l RON95. That gives me an average of 6.67l/100km. The thing is we were 4 on the car, plus luggage, and for most of the time I drove at 140km/h! It took us around 3h30m to complete the trip, because I couldn't drive that fast all the time.
I've noticed that above 150km/h the "mileage pointer" (under the tachometer) was always above the 10l/100km, but at 140km/h it floated between 4l to 8l (depending on the road slope).
If you imagine a car driving @70km/h on top of other one also driving @70km/h on the same direction,this means that I should do a 3.3l/100km average on my Aveo, driving @70km/h, as I usually do.
I was convinced that speeding kills the mileage, that fuel consumption was exponential above 70-90km/h. Apparently not...