Oh that one !

I'd have thought it ought to do better than 100 mpg ! Volkswagen have
already done their '3 litre car'. That is it'll do 100km on 3 litres of
fuel. (78 US mpg).
http://en.wikipedia.org/wiki/Volkswagen_Lupo#Lupo_3L

Note the absense of energy sapping stuff like a/c though.

And then there's their 2 seater 238 mpg concept car.
http://en.wikipedia.org/wiki/Volkswagen_1-litre_car

Graham