I remember seeing the aricle on this car 450mpg was achieved on 4.78 oz of fuel!
the car was a 1959 Opel Rekord 2 door saloon converted to a Ute (it may have had a fibreglass body) and the event happed in 1974 by Ben Viser see Article : May 1974 Australia Motor Manual Page 16
The car was also featured as Headline display at the 1975 Amsterdam Auto Show
see photo in 22 Feb 1975 Autocar (GB) Page 54 Volume 142 Issue #4087
General belief is it may have been fitted with a special carb made by a company called Economy Motor Co of the USA - The Story goes that guy before world war 2 invented a new type of carb that got amazing results - (this could be BS and typical tall story!) and he tried to market it and he was visited by the guys in black suits who put the usual threats for him not to market it ;
(after all even here in NZ Archie Blue in 1977 with his water powered Mini was told by NZ officals not to market his car ...this was reported in a article with reprint of 1977 Newspaper story by The Constructors Car Club Club Mag 'Spare Parts' in 1995)
but then US Govt stepped in and apparently a little known scarily US Law - that anything invented that upsets the US Govt interest can be taken and no royalities paid - The Govt can remove the item and take up ownership .. and poor guy never got any royalities for it as carbs were put on US tanks during the war years! Once tanks were sold these carbs were removed and I guess stored?
In addition with regard to the Opel - I heard a book was produced by SAE of the US as limited run in the 1960s now out of print that discusses radical designed carbs that get amazing results and this book is highly sought after today by SAE Engineering students and staff around the world!