I generally agree with you Saab1... but at the moment, since the car is new, I was keeping a close eye on consumption (both from the computer, and measured the old fashioned way) to see how accurate the computer was, and what difference there is to my old car.
I also find that however I drive - within reason! - my mileage never moves more than +/- 1mpg... unless I do a long motorway drive, for example.
I was watching carefully because of general running problems.
However, the answer was more prosaic (and embarrasing ) ... a slow puncture causing a flat tyre! Doh... It did mean there was a problem, just not the one I was looking for .