I recently bought a new VW Golf 1.6 tDi 105 ps and have managed a best of 82.5mpg on a trip between Bingley and Criccieth in N Wales. But my word, it is hard work travelling at 50mph on motorways, watching out for lorries overtaking.
However I can do 75mpg on the same trip at 60+mph. It is an amazing engine and powerful as well, if you want or need it.

I can get good results from an old 1997 Audi A6 2.5 TDi. I can get up to 60 mpg on long runs at around 70 mph. Its actually worse at less than 60mph because of the tall gearing. It makes the Green party's idea of a 50mph motorway speed limit a bit of a nonsense, but I sense its an idea to make driving 'boring' and less desirable rather than efficient. Tall gearing on all cars (i.e. a sixth or seventh gear) would be a better policy.

There does need to be more education on smarter driving and its certainly not about driving like 'Albert and Ethel' on a Sunday. Its about getting smooth speeds, anticipating lights changing and traffic at junctions and minimising braking and harsh acceleration, but its also about not going too slow for corners etc (within reason of course!).

Occasionally, however, you do also need to do the opposite to avoid excessive carbon build-up in you engine which will have a bad impact on mileage and performance. So it is good (and equally fun) to give your car an 'italian tune-up' every now and again for a few miles to blow the cobwebs, usually by accelerating full load up to the red-line a few times.