I've always found this as well, driving style doesn't make much difference at all. In fact, I seem to average ~55mpg on any diesel I've driven, thats 2.0 HDi 90/125/130/135, 2.0 TDi 140, 1.9 CDTi 150, 1.9 TDi 130/180, 1.6 TDCi 110/115/140... Even being mapped doesn't make any difference, the only time I notice a difference over a whole tankful is if I do a lot of stop start town traffic...and there's nothing you can do to make that efficient! I even have stop start on the current Focus but it doesn't save any noticeable fuel in traffic. I know it's meant for emissions reduction in urban areas rather than fuel economy though.
Petrols are a different matter though... Keep thrashing those and the MPG drops off quickly!
Petrols are a different matter though... Keep thrashing those and the MPG drops off quickly!