It's a matter of torque and gearing. Quite a lot of diesels, for example the Peugeot/Ford 2.0TDCI, develop maximum torque at 2000rpm which corresponds to 70mph in sixth, which can actually make the car a bit fuel hungry at 70mph. The chain cam 95ps FIAT 500 1.3 MultiJet could be your best bet, delivering 60 - 65mpg at 70mph and with surprising comfort from the 2010 model suspension and excellent seats. Others claim astonishing mpg figures in the EC tests, but I have yet to match them in real life. And a £500 timing belt change every four years or 40,000 miles on a VAG diesel has to be set against the economy benefit anyway.