PSA (Citroen and Peugeot) are ahead of the game in Europe and their engines are used by Ford, Land Rover, BMW (Mini), Volvo and others.
The HDi engines are very efficient - my large family car, a Citroen C5, does 60mpg and has even managed 70 mpg on a long drive to Bordeaux from Manchester.
They have one drawback and that is the cambelt. You need to change it in line with the service manual at 100,000 miles (not expensive to do) otherwise it can break and destroy the head.
My car has done 113,000 and I have owned Citroen Xantias that have covered 250,000 miles. There is a Xantia taxi in Manchester that has done 500,000 miles and is still going strong and returning 50 mpg - with the added advantage that it can run on straight vegetable oil.
The US needs to turn it's money and engineering might to fuel efficiency - come on guys, show the rest of us what the US can achieve, take us into the NEXT century!
And as for The Green Car of the Year - 21mpg, you are joking right!