Thanks Gremlin. That seems to be making sense. So my next question would be, what happens when you hit 4000 rpm?
Of course, the car is running between 1400 and 4000 rpm 99.9% of the time, so it's safe to assume that the turbo never stops.
Personally, I tend to stick to 70 mph on the motorway, not because that's the legal limit, but above that I see the mpg readout fall well below my average mpg reading. I think that's about 2500 rpm in 5th gear isn't it?