There are cars you buy for economy and those that you don't you don't buy anything bigger than a Micra or a small diesel family car if you want to maximise your fuel consumption. A Zetec Cougar as a money saving exercise is a waste of space, but you find the same thing with a lot of cars. Why on earth, for example, is there a choice of 1.8 and 2.0 Mondeo? What a friggin waste of time, other than the psychological effect some people need from sensibly going a few cc less.
I just don't bother looking. I do a fairly large amount of milage and figure that my total motoring costs will be hefty anyway. I can trade off road tax and fuel costs against the massive maintenance and repair costs of modern diesels as well as depreciation by getting something a few years old, and just enjoy myself rather hating my life because I hate my diesel and generally being miserable.