The World Solar Challenge in Australia this year featured a Greenfleet Technology division for high mileage alternative fuel vehicles. One of the top competitors was the Audi A3 TDIe powered by a 1.9L four cylinder diesel. The team of drivers covered the 3,543 km distance from Darwin to Adelaide Australia with an average mileage of 71.3 mpg (US) with peaks as high as 90.4 mpg. The special "e" series A3 is tuned for maximum efficiency and was introduced to Australia at the Sydney Motor Show. It goes on sale Down Under later this year.
[Source: Fourtitude via GermanCarBlog, thanks to Christian for the tip]