I have a recent diesel (2010 VW) and to be honest I'm disappointed with the fuel economy. I've driven diesels for years, all around 1700cc but mechanically injected and have always had 53MPG on all journeys. This VW, common rail/electronically injected is widely variable and so averages only 48 MPG not the consistant average MPG in the high 50's I was expecting. I've seen as much as 63MPG over a 250 mile journey and little as 25MPG around town.
Alas, there isn't some sort of law for fuel economy like there is for the speed[computing power] of computers (doubles approximately every 2 years) - I recall finding a newspaper page from the 70s that was an advert crowing about how the new renault line up would have the arab oil boys "sheiking" with rage at how economical they were - a 1.4 carburetted renault 5 campus getting mid 40s ..
What IS amazing, however, is how the modern cars manage to deliver the performance and economy they do while farting out sunshine and glitter rainbows, compared to the engines of yore..
If you want superb economy, though a little uncivilised compared to the cossetting cocoon of a modern car, you're going to have to throw your leg over a 4 stroke 125 motorcycle for the town bimble. 100mpg is easy to achieve and if everyone did it, a vast majority of traffic problems would evaporate.. Not everyone will do it, but even if they don't, for your own purposes riding is a better way to get through congested streets than driving.
The fly in the ointment of course, is that this country doesn't really get on with the idea of stuffing three kids plus rider on a tiny motorcycle to do the school run, unlike some other countries that depend massively on a huge motorcycle population to keeps things moving. India,for example, would be screwed if car ownership was even 20% higher than current levels